[1.x] [extensibility] Add (some) missing shims

[DavideIadeluca]

Fixes #0000

Changes proposed in this pull request:
Add some missing Typescript Shims

Reviewers should focus on:
I only added some of the lower hanging fruits, which are most likely to be used in third-party extensions and don't require a TS refactor of existing code. Some of the work on this has already been done on the 2.x branch.

Is this PR something that's already valuable enough to the ecosystem, or would you prefer a more thorough PR which aims to add everything possible?

Screenshot

QA

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.

Required changes:

• Related documentation PR: (Remove if irrelevant)
• Related core extension PRs: (Remove if irrelevant)