New (forked) version with enhancemenet

Hi @alexadam ,
In the last three years, in my spare time, I worked on your extension fixing bugs and add features. At the beginning, my idea was to keep it for myself (ok, it's on github but I have never promoted it), but the last two features I've implemented, namely Mozilla Reader View integration and Manifest V3 support, are quite useful and I've decided to share it with others.
In shorts, I've implemented the following features with respect to the original extension:

1) Manifest v3 support (both for Firefox and Chrome)
2) Mozilla Reader View integration
3) Font size relative to the page (quite useful when reading via an actual ebook reader)
4) Removed useless dependencies. Now the code dependencies are only JSZip, DOMPurifier, and Mozilla Reader View
5) Import/Export custom css rules
6) Better iframe support
7) More HTML elements are supported

Since this project is not currently developed, I can PR you with all my changes (just a single PR, not a PR for each change, they are built on top of each other). I'm 79 commit ahead of the master repo, but still the code still resemble your master branch. I planned to have a complete refactoring/rewrite, but it takes too time.

Let me know if it's okay.

Cheers!