Add isRepoFile404

[134130]

Description

• is404() can not detecting new 404 pages
  • https://github.com/fregante/GhostText/tree/3cacd7df71b097dc525d99c7aa2f54d31b02fcc8/chrome/scripts/InputArea
  • https://github.com/refined-github/refined-github/blob/some-non-existent-ref/source/features/bugs-tab.tsx
• related:
  • useful-not-found-page broken on new 404 repo page refined-github#6413

[fregante]

The original 404 page had zero UI so the whole extension was disabled, with the exception of useful-not-found-page.

These new 404 pages are actually just repo pages that have a "not found" area, so they still benefit from a host of other repo-wide features like bugs-tab and ci-link

To fix refined-github/refined-github#6413, we need to have a separate isFile404 detection, perhaps, and then add a new init in useful-not-found-page to handle this new view.

[fregante]

Generic
File

[134130]

I'll fix this branch as detecting isFile404