refactor: unify frontend initializers naming

[DavideIadeluca]

Fixes #0000

Changes proposed in this pull request:
Use consistent naming for frontend initializers (and slightly change syntax), as the current best practice seems to be acme-foobar. Some bundled extensions were still initialized like this: acme/foobar or foobar.

Reviewers should focus on:
Seems like no bundled extension does any check in the frontend if another extension is initialized, for example:

if (app.initializers.has('flarum-tags')) {
    ...
}

So, as far as I can tell, the changes introduced in the PR should work standalone. But please double-check that I've not missed anything else for this to work out of the box (I have not yet setup 2.x for local development..)

Extension developers need to refactor their frontend initializer checks when upgrading their extensions for 2.x, so this must be included in the upgrade guide and potentially also noted as a breaking change.

Screenshot

QA

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)