Restructure as a valid R package

[christopherkenny]

This PR restructures theme_park into an R package. It does a lot, so happy to chat over details before any merging. Primary changes are listed below.

These files are generated by default as part of initializing an R package with devtools::create()

• DESCRIPTION, which contains the key details of the package, like name and licensing. Some details here will need to be drafted and added, like a title (can't just be the same as the short name) and a description (must be a few sentences describing the package).
• an Rproj file, themepark.Rproj
• .Rbuildignore, a listing of files for R's package building to ignore, similar to a .gitignore file
• NAMESPACE, a file that lists out exports and imports

The next big bucket of changes is moving the .R files into the R/ folder so that the package knows what to use.

• example.R at this point is excluded, as that will be incorporated into README.Rmd.

On the topic of README.Rmd, many packages use that now as it will let you put examples in the readme without doing anything crazy.

• this file integrates the content of example.R and README.md, so you get both the instructions and the examples without having to actively chase the examples to make them work
• devtools automatically includes a pre-hook commit that tries to makes sure you're editing in the right place (the .Rmd rather than the .md)

With the basic structure, we can then start adding package content.

• Rather than requiring packages with require(), these are now in the Description
• documentation goes into the man/ folder
• at this point, we have to move the theme_template.R file outside of the R folder, as it is something we want to be able to use as a resource but is not valid R code. (In theory, we could also just Rbuildignore it, but making it an explicit resource is helpful later on).

Then beyond moving out of the R folder, we need to make a series of changes to the existing R folder

• first we want to move the utility functions into a utils.R file. (this can be renamed to anything you prefer.)
• then we need to clean up the showtext stuff so that it
  • isn't reloaded as if it's a script, so we move the showtext part into onLoad in zzz.R
  • fonts are added within themes, rather than running code on load.
    • to make this fast: (1) fonts are only added if they aren't already available and (2) fonts are only added if XXXX_font is TRUE
    • see initialize_font source code in utils.R

Finally, in utils.R, I've added new_theme_park_theme() which makes use of the template to create a new theme. This can be edited to add in any new constraints or checks.

Some near-term action items

• [x] I used themepark since underscores can't be included. This can be changed, for example to theme.park or themePark depending on your preferences
• [x] The description needs a title that differs from the package name, as it'll be displayed as pkgname: Title
• [x] The description also needs a description. This is typically a single paragraph explaining what the package does and what it contributes to the R community.
• [x] How can we make exported colors sustainable? If each template needs to export 6-10 colors, then that becomes a problem by the time the package has about this many templates? In other packages, I've seen exported lists with sets of colors as entries. So you might have a list that has names like nemo or godfather and then has named entries within. See #18 for discussion.
• [x] The existing themes should be restructured into the package template. Happy to start adjusting those, but that seemed like too much for one PR. Those changes should likely be done in a branch and PRed into this BEFORE merging this into main.

Some later action items

• Once things are looking good, @MatthewBJane should run usethis::use_github_action_check_standard() to init the GitHub action checking
  • doing this too early will get annoying though, as any failures generate messages and it's likely to fail R CMD check until the near-term action items are handled
• (OPTIONAL, but helpful) @MatthewBJane can run usethis::use_pkgdown_github_pages() to give a default pkgdown site
• Updating the PR/issues template to suggest using the new functions and giving some standards for what things people should change (such as adding their name to the README and package description as a 'ctb' (contributor)).
• Once the structure is ready, it's worth adding tests (even if they're super simple and just checking some return types or error messages). This can be added with usethis::use_testthat()

[MatthewBJane]

Wow great job, Christopher! I am ready to merge when you are! I'll answer the near term items below:

• I think my preference is to name it ThemePark
• The description title can then be Theme Park (if spaces are allowed)
• Description: The Theme Park package stores ggplot themes that mirror works from popular culture, such as Barbie, Star Wars, Game of Thrones, etc. The package currently holds N themes and a number of corresponding discrete color scales, palettes, and fonts. Each theme (e.g., theme_barbie) generates a unique color scheme and font for a ggplot object that matches the color scheme and font found in the movie, tv show, video game.
• I addressed the color point in your issue
• I can work on this when I have some free time this weekend.

[christopherkenny]

• Okay, great. That's updated.
• The title isn't supposed to repeat the package name, as it will be displayed as "ThemePark: Title". They tend to be more like "Themes for 'ggplot2' from Popular Culture". I put that in for now, but obviously it can be changed.
• Added with very minor edits. (There are some weird patterns that CRAN doesn't allow, such as "The package ..." at the start, though it's okay later on. If you're interested in details, Hadley Wickham's book has a great explainer.)
• Your choice makes sense to me.
• That point was resolved by #19, so we're good to go.

[christopherkenny]

Colors point is resolved. Good to merge by me.