feat(jest): create jest config package for unit testing

[SychO9]

Changes proposed in this pull request:
This is part one of two PRs to introduce tests to our JavaScript codebase. This PR lays the foundation by introducing a flarum-jest-config package and the necessary changes to begin unit testing. An example core abbreviateNumber util test is added as well.

Reviewers should focus on:

• Anything I might be missing about introducing Jest testing.

Part two is here: #3679

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)

[SychO9]

need to add the workflow

[askvortsov1]

Wonderful, thanks so much for making this work! I think the only uncertainty I have is whether it would be better to put test code next to the source code, either in the same folder or in a __tests__ subdir. That way, it's easier to keep track of source code having corresponding tests, although coverage should help with that too.

[askvortsov1]

A lot cleaner, yay!

[SychO9]

Wonderful, thanks so much for making this work! I think the only uncertainty I have is whether it would be better to put test code next to the source code, either in the same folder or in a tests subdir. That way, it's easier to keep track of source code having corresponding tests, although coverage should help with that too.

I don't have any strong opinions about this at the moment. I'm okay with it either way. We will probably find out as we write more tests in the future.

[askvortsov1]

Let's keep as is for now then, LGTM!