feat: introduce frontend extenders

[SychO9]

Changes proposed in this pull request: Prior to this, frontend extenders never actually worked before, nor could any extension use them at all as they weren't exported in compat either. This PR does the following:

• Refactor extenders in core for them to actually work:
  • They are now in the common/extenders directory to avoid conflicts with the extend.ts module).
  • An IExtender interface has been introduced.
  • Th Routes and PostTypes extenders have been adapted to the new changes and the Model extender was removed for a separate PR.
• Make use of the Routes and PostTypes extenders in bundled extensions.
• The extenders can be accessed by exposing an extend module from the extension, which contains an array of extenders.

Reviewers should focus on:

• JS Tests are only failing because check-typings is running before new typings are built.
• The bundled extension changes were made here as an example, but I can extract them into another PR if wanted. The commits are separated though, so it should be easy to review.

Necessity

• [ ] Has the problem that is being solved here been clearly explained?
• [ ] If applicable, have various options for solving this problem been considered?
• [ ] For core PRs, does this need to be in core, or could it be in an extension?
• [ ] Are we willing to maintain this for years / potentially forever?

Confirmed

• [ ] Frontend changes: tested on a local Flarum installation.
• [ ] Backend changes: tests are green (run composer test).
• [ ] Core developer confirmed locally this works as intended.
• [ ] Tests have been added, or are not appropriate here.