[imorland]

Fixes #3758

Changes proposed in this pull request:
Toggling an extension only flushed the formatter cache when that extension registered Extend\Formatter itself. Extensions that configure the formatter based on which other extensions are enabled were left with a stale formatter: after enabling tags, #tag was saved as plain text, and after disabling it, replying with #tag failed with a 500 until php flarum cache:clear was run. Core now flushes the formatter whenever any extension is enabled or disabled, like it already does for the frontend assets and locale caches.

Reviewers should focus on:

• The test fires Enabled/Disabled directly: the testing harness pins the enabled extensions per test case, so a real toggle can't change isEnabled() inside a test. The tags/mentions case was checked by hand on a dev forum, both directions.
• Multi-server setups with separate local storage still only flush on the server that handled the toggle; that already applies to cache:clear and isn't changed here.

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

• Backend changes: tests are green (run composer test).
• Backend changes: tests have been added, or are not appropriate here.
• Core developer confirmed locally this works as intended.
• The description above is written by me and describes what this pull request actually does.