perf: transpile js to es6

[davwheat]

Changes proposed in this pull request:

• output ES6 Javascript instead of ES5

All extension JS source code will need to be recompiled with an updated Webpack config.

This will reduce our tech debt and overall bundle size due to unneeded ES5 polyfills included by Babel despite us not supporting any ES5 browsers.

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]

@davwheat is this complete?

[davwheat]

Yeah, I think so. I only marked it as draft because we weren't 2.x-dev ready then.