feat: display a warning when not on Windows

[leon-richardt]

Description

Closes #49.

Since only Windows is supported at the time of writing, it makes sense to be display that information when users try to run the extension on anything other than Windows.

This information is also available on the extension settings page, but much more discoverable here.

Screenshot


Additional Information

Would be great if someone could test this on Windows and Mac as well. Code criticism is very welcome as I rarely work with JavaScript and this is my first time hacking away at a browser extension.

[ALazyMeme]

[ALazyMeme]

@pajlada review pls 😄