chore: better typings for DiscussionListState

[davwheat]

Changes proposed in this pull request:

Explicitly define function return types in DiscussionListState.

Defining these help to identify changes that inadvertently change the return type, and are this breaking.

Reviewers should focus on:

Are the requestParams typings correct?

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)

[askvortsov1]

Could we enforce that if q is present in filters, it should be the only key?

[davwheat]

Ooh, that's a good question. Let me have a play.

[davwheat]

I couldn't find a way.

One of the promising things was:

filter: { q: string } | { [key: string]: string } & { q?: never }

but this seemed to not work correctly, for some reason.

[askvortsov1]

I couldn't find a way.

One of the promising things was:

filter: { q: string } | { [key: string]: string } & { q?: never }

but this seemed to not work correctly, for some reason.

Fair enough, let's keep things as is then