chore: unify JS actions into one (rewritten flarum/action-build)

[SychO9]

Relies on flarum/action-build#10

Changes proposed in this pull request:
Unifies all frontend workflows into one.

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.

[SychO9]

Tested on a single extension repository to make sure things are still good on that end ✅

[SychO9]

Bundlewatch is disabled for now since we use a single frontend workflow. Maybe we can specifically have it only measure core dist files, or if possible all dist files at once?

[davwheat]

I think disabling Bundlewatch for now is the best. We can look into re-enabling it later.

[davwheat]

Rebased to (hopefully) pass PHP tests.

[SychO9]

Here goes nothing..