feat: Code Splitting

[SychO9]

Closes #3857

Changes proposed in this pull request: Read the plan laid out in the issue to understand the changes introduced here.

Necessity

• [x] Has the problem that is being solved here been clearly explained?
• [x] If applicable, have various options for solving this problem been considered?
• [x] For core PRs, does this need to be in core, or could it be in an extension?
• [x] Are we willing to maintain this for years / potentially forever?

Confirmed

• [x] Frontend changes: tested on a local Flarum installation.
• [x] Backend changes: tests are green (run composer test).
• [x] Core developer confirmed locally this works as intended.
• [x] Tests have been added, or are not appropriate here.

Related PRs

• https://github.com/flarum/docs/pull/461

[SychO9]

Should work on windows now

[SychO9]

Ignore backend failures, will deal with those separately.