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>

jsconfig.json

@@ -5,6 +5,7 @@
     "module": "esnext",
     "moduleResolution": "bundler",
     "experimentalDecorators": true,
+    "allowJs": true,
     "paths": {
       "discourse/*": [
         "./app/assets/javascripts/discourse/app/*"