fix(webpack-config): split chunks can fail in dev mode

[SychO9]

Changes proposed in this pull request:

• When running yarn dev webpack changes certain behavior that can break spit chunks.
  • In the example where TagDiscussionModal extends from TagSelectionModal and both components are split chunks. The normal wted behavior in yarn build is two separate chunks where TagDiscussionModal also has the TagSelectionModal component inside. In yarn dev webpack attempts to speed up its process by just async loading TagSelectionModal first. Since our internal process can only handle explicit async loading (to determine the source package/extension), this behavior errors out.
  • By configuring webpack's auto async calls to a max of 1, this is prevented.

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.