feat: extension list UI

[SychO9]

Part of the roadmap

• Adds an extension list UI by communicating with flarum.org api.
• The design follows the mockups submitted in: https://discuss.flarum.org/d/34189-flarum-20-extension-manager-uiux-for-discovery-and-installation
• Unifies pagination (User list, tasks queue, extensions list) into a singular new Pagination component.
• Makes the user list loading container usable elsewhere.
• Extracts the Search modal tabs into a reusable Tabs design classes.

Screenshot

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)