[WIP] Add "theme_experiment" addon support

[trickypr]

What is this
This is an implementation with the goal of closing #143. You can find more information by reading it.
tl;dr: There is a way of distributing themes as extensions to Firefox Nightly and Firefox forks.

Specific details
This adds the ability for a user to select the browser they are using and be provided with install instructions for using "theme_experiment" addons within their browser. There is currently no way to have themes that only support "theme_experiment" but themes without "theme_experiment" are not impacted in any way.

Checklist

• Themes can be added to themes.json
• The tests are updated to ensure they work as intended (depends on xpi-creator)
• "theme_experiment" addons have correct install instructions
• Method for filtering for only "theme_experiment"
• Instructions for theme developers to package
• Discussion around the UI / UX of this implementation

Live demo: https://trickypr.github.io/FirefoxCSS-Store.github.io/

[BeyondMagic]

I plan to work on this, but one thing I noticed already is that this PR changes the way we used to build files and it doesn't work properly at all. After #219, I'll take the proper code of this feature and transpile to Astro.

Anyway, thanks @trickypr for working so far in this feature.

[unseen-ninja]

I think this actually could be pretty easy – we can tag all themes accordingly and just generate the static files according to wheter the tag is present or not. This would again remove any computational stuff client side.

[trickypr]

I probably won't continue working in this. If you want to steal any of my code, feel free.