- modernized the code base, fixed all linting issues based on new development standards
- updated webext from version 1.6.0 to 6.6.0
- bumped the minimum required Firefox version to Firefox 91

* Rewritten as WebExtension (so it will be compatible with Firefox 57 and higher)
* Changed development environment from jpm to web-ext (version: 1.6.0)
* The button changes to an inactive state if no feed is available
* improved feed detection