FEATURE: introduce a ProseMirror editor (#30815)

This is the first in a series of PRs to introduce a
ProseMirror-based
WYSIWYM editor experience
alongside our current textarea Markdown editor.

Behind a hidden site setting, this PR adds a toggle to the composer
toolbar, allowing users to switch between the two options.

Our implementation builds upon the excellent ProseMirror and its
non-core Markdown
module, using the
module's schema, parsing, and serialization definitions as the base for
further Discourse-specific features.

An extension API is included to enable further customizations.

The necessary extensions to support all Discourse's core and core
plugins features **will be implemented in subsequent PRs**.

---------

Co-authored-by: David Taylor <david@taylorhq.com>

config/site_settings.yml

@@ -3554,3 +3554,7 @@ experimental:
   use_overhauled_theme_color_palette:
     default: false
     hidden: true
+  rich_editor:
+    client: true
+    default: false
+    hidden: true