In the fallback code for native key bindings and in the inline table handling,
the editor still use `GetDOMEventTarget()` and `IsInUncomposedDoc()` which are
not aware of shadow DOM.
Differential Revision: https://phabricator.services.mozilla.com/D193621
This enables the editor directory to be linted, and fixes the remaining issues raised by ESLint. Various rules were fixed here including, no-shadow, no-undef, no-unused-vars and others.
I've generally gone conservative, disabling rules where it doesn't make sense to fix them (e.g. sometimes suggests use-services for tests, but it is only used once, or within a Chrome script).
Depends on D5585
Differential Revision: https://phabricator.services.mozilla.com/D5587