[mv3] Add old-school filter editor for filter list authors

The filter editor is in the "Custom filters" pane in the dashboard,
and is available only when "Developer mode" is enabled.

The filter editor is also only available in browsers which
WebExtensions framework supports the `userScripts` and `offscreen`
APIs (so only Chromium-based browsers currently).

The filter editor's main purpose is as a tool for filter list
authors, it's not meant to be a replacement for importing filter
lists due to the maxium number of dynamic DNR rules allowed.

This is a first version with a list of TODOs, and some filter
syntaxes are currently not supported:

- not possible to `badfilter` a filter
- cosmetic exception filters do not apply to stock lists
- generic cosmetic filters are currently discarded
- `popup` filters are discarded
- `doc` filters can't trigger the strict-block page