Add new Material and Ayu themes

[gadenbuie]

Moved the PR #47 by @lusignan into a new PR branch to pick up new changes in rsthemes. Also remaining tasks are mostly documenting and reviewing.

• [ ] Document new themes and original palettes
  • [ ] https://github.com/ayu-theme/vscode-ayu
  • [ ] https://draculatheme.com/
  • [ ] https://material-theme.site/
• [ ] Use upper tab border in Ayu themes
• [ ] Use thin upper tab border in Dracula
• [ ] Review color themes

[gadenbuie]

@lusignan how did you source the Material theme palette? While testing, I noticed some differences in the colors I see in VS Code and the ones used in the new rsthemes. I found two places where the material colors are described

1. https://github.com/material-theme/vsc-community-material-theme/tree/develop/themes
2. https://github.com/material-theme/vsc-material-theme/blob/master/material-theme.config.json

but I'm not sure if you used those or found a better resource.

[lusignan]

@gadenbuie I generated a json of the theme from VS Code's command palette (identical to the first link you posted) and then used the hex codes from that to build the rsthemes version. The hex code for gold looked more orange in RStudio than it did in VS Code, however, so I modified that for the rstheme. The backgrounds also looked different in RStudio so I used Chrome's "ColorPick Eyedropper" extension to find a closer match.

The hex codes in the second link are for the accent options in VS Code's UI but are not colors used in the syntax theme.