feat(tags): admin tag selection component (reusable tag selection modal)

[SychO9]

Replaces #3495

Changes proposed in this pull request:
A lot of times you need a way to provide a tag selection component in extensions, especially the admin settings. This PR:

• Extracts a reusable TagSelectionModal from the existing TagDiscussionModal so it can be used in different contexts.
• Adds a tag selection admin setting component which uses the new generic modal.

Reviewers should focus on:

• The real changes happen between TagDiscussionModal and TagSelectionModal.
• Custom setting component general code quality.
• KeyboardNavigatable was moved to common, should we keep a KeyboardNavigatable export in forum? I currently kept a deprecated compat export only.

Code Snippet

app.extensionData
    .for('flarum-acme')
    .registerSetting({
      setting: 'flarum-suspend.tags',
      type: 'flarum-tags.select-tags',
      label: 'Relevant Tags',
      help: 'Lorem ipsum dolor sit amet.',
      options: {
        requireParentTag: true,
        limits: {
          max: {
            primary: 2,
          },
          min: {
            primary: 1,
            secondary: 1
          }
        }
      },
    })

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.