[2.x] feat(core, mentions): implement ability to order groups

[DavideIadeluca]

Fixes #0000

Changes proposed in this pull request:

• Implement ability to change ordering of Groups, similar to how it works for flarum/tags
• Implement reusable GroupBar component which contains the sortablejs integration. Extensions like https://github.com/imorland/flarum-ext-twofactor/blob/2.x/js/src/admin/components/ExtractedGroupBar.tsx could consume this new component

Reviewers should focus on:

• Backend, how to handle the Guest and Member Group here? The DB migration does set a position, but the first time the order is changed in the frontend those groups' position would be set back to null. From a usability point of view it doesn't matter, but it could be argued it's a quirk we don't wanna have
• Also the changes in the webpack config.. it appears like that the current minification step from Terser fails to parse the esm build of sortable js due to unsupported syntax. The alias I added forces webpack to use sortables UDM build which appears to fix the issues. Really unsure how we should address this issue and if maybe changes in flarums webpack config itself are needed.

This was the error message (reproducible both in CI and when running yarn build locally):

ERROR in admin.js
  admin.js from Terser plugin
  Unexpected token: punc ({) [webpack://../node_modules/sortablejs/modular/sortable.esm.js:1212,31][admin.js:22591,37]
      at js_error (/home/runner/work/framework/framework/node_modules/terser/dist/bundle.min.js:536:11)
      at croak (/home/runner/work/framework/framework/node_modules/terser/dist/bundle.min.js:1264:9)
      at token_error (/home/runner/work/framework/framework/node_modules/terser/dist/bundle.min.js:1272:9)
      at unexpected (/home/runner/work/framework/framework/node_modules/terser/dist/bundle.min.js:1278:9)
      at semicolon (/home/runner/work/framework/framework/node_modules/terser/dist/bundle.min.js:1316:56)
      at simple_statement (/home/runner/work/framework/framework/node_modules/terser/dist/bundle.min.js:1577:73)
      at statement (/home/runner/work/framework/framework/node_modules/terser/dist/bundle.min.js:1390:19)
      at _embed_tokens_wrapper (/home/runner/work/framework/framework/node_modules/terser/dist/bundle.min.js:1329:26)
      at block_ (/home/runner/work/framework/framework/node_modules/terser/dist/bundle.min.js:2168:20)
      at _function_body (/home/runner/work/framework/framework/node_modules/terser/dist/bundle.min.js:2080:21)

Screenshot
https://github.com/user-attachments/assets/8dcb047b-bd52-435e-a5b1-a5da46673664

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)