chore: replace for ... in with Array.reduce

[davwheat]

Changes proposed in this pull request:

Replace for ... in loop with Array.reduce.

Reviewers should focus on:

It's a reduction in bundle size (one whole byte!!), and I think it benefits from the readability and helps to set our precedent for more modern helper methods within Flarum.

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.

[askvortsov1]

Now do it in OCaml

[davwheat]

@askvortsov1 yeah think I'll pass on that one... 😂