Invert dark favicons wot spaniol

Known bugs:
- The extension doesn't work well in some websites (such as chatgpt.com), resulting in black favicons when it shouldn't. For those, disabling the extension via the popup menu usually fixes the issue

Planned features:
- White-on-white mode for users of light themes

Please report any bugs by opening an issue on:
https://github.com/nicolasspaniol/firefox-invert-dark-favicons