Support, update to webpack 5

[askvortsov1]

**Fixes #2855 **

Changes proposed in this pull request:

• Support webpack 5 by separating JS entries with semicolons
• Update core to use webpack 5
• Update core to use expose-loader v2

Reviewers should focus on:
This also requires changes to flarum-webpack-config. These are trivial, so I've pushed them directly to main, but I've tagged the version as 2.0.0-beta.2 pending review. Once this is approved, I'll bump / publish that as v2.0.0, update this PR to use that, and then merge.

https://github.com/flarum/flarum-webpack-config

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.

[davwheat]

Isn't this breaking, and should be saved for 2.0? I remember some issues getting Webpack 4 + 5 to work together, or are these not an issue anymore?

[askvortsov1]

Isn't this breaking, and should be saved for 2.0? I remember some issues getting Webpack 4 + 5 to work together, or are these not an issue anymore?

The problem is if you have v5 extensions with v4 core. v5 core with v4 extensions should work fine

[davwheat]

And it even reduces our bundle size! 🎉