chore: update JS dependencies

[SychO9]

Fixes #3681

Changes proposed in this pull request:
To update dependencies we first lock mithril to 2.0 as 2.2 contains breaking changes. We can look into once they release a migration guide.
We also update some typing-specific changes needed as tsc has been updated.

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)

[imorland]

Seems fine locally 👍