Allow using all of github's themes

[jjspace]

Summary

• Utilize the updates added to generate-github-css to include options for all theme types in a new generation script
• Modify the included stylesheets to apply based off of data attributes instead of a single class name to allow for easier mixing and matching. (This matches up closely with how Github themselves does it)
• Expand logic for selecting a theme to allow specifying the light and dark themes to use when in Auto, System and Single modes
• Add settings for Light an Dark themes separate from one main theme setting
• Changed project over to a type: module to allow working with generate-github-markdown which is an ESM module not a CommonJS module. This also required changing the extension.js to extension.cjs to continue working as it used to
• Updated the Readme to reflect the new settings (and also address a question I saw in the issues list)

I believe the way the new settings are set up will preserve the existing behavior for anyone who had modified them before.

Resolves #89 Resolves #75

Please let me know if you have any questions!

I don't know much about actually publishing extensions so I believe all my changes should work for those who install them but please verify for me so I'm not accidentally breaking this for a bunch of users (myself included)

[mjbvz]

Great work! Looks like a very nice addition. Will test a little more and then publish shortly if everything looks good

[jjspace]

@mjbvz I'm sorry for accidentally creating some extra issues with my changes and making you do extra cleanup (https://github.com/mjbvz/vscode-github-markdown-preview-style/issues/104, https://github.com/mjbvz/vscode-github-markdown-preview-style/issues/103). Can you elaborate a little on why your fixes were needed/the better changes in these cases? (e0286fd5580cf39704a3d3a6666919573c1b04de b2adb1fbadda6957888345b32a55594f218746a5, d01f1e7a1d39f4210572086132ea53c9e255f5e0) Just so I can learn and don't repeat mistakes in the future as I've never worked on vscode extensions before (and "it works fine on my machine" is not a great excuse :laughing:)

ps. using this pr just to avoid excess noise for other users in the issues, wasn't sure the best place for a discussion

[mjbvz]

No worries, the fixes were pretty quick and easy (and the more important break was my fault)

To recap. The first issue was that .cjs doesn't seem to work on web. I fixed that but in the process broke this extension on desktop because I forgot to commit the change that also removed type: "module" in the package.json. type: "module" caused the extension.js script to be loaded as a module, which was now failing because it's commonjs and no longer had a explicit .cjs extension to tell node otherwise. Then finally I had to make sure the build script was treated as a module again by converting to an .mjs file

Probably not a common issue. Will see if I can add a browser debug config to the extension so you can more easily test the extension in browsers