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>

spec/system/page_objects/components/composer.rb

@@ -5,6 +5,12 @@ module PageObjects
     class Composer < PageObjects::Components::Base
       COMPOSER_ID = "#reply-control"
       AUTOCOMPLETE_MENU = ".autocomplete.ac-emoji"
+      HASHTAG_MENU = ".autocomplete.hashtag-autocomplete"
+      MENTION_MENU = ".autocomplete.ac-user"
+
+      def rich_editor
+        find(".d-editor-input.ProseMirror")
+      end
 
       def opened?
         page.has_css?("#{COMPOSER_ID}.open")
@@ -113,6 +119,14 @@ module PageObjects
         page.has_css?(".discard-draft-modal")
       end
 
+      def has_hashtag_autocomplete?
+        has_css?(HASHTAG_MENU)
+      end
+
+      def has_mention_autocomplete?
+        has_css?(MENTION_MENU)
+      end
+
       def has_emoji_autocomplete?
         has_css?(AUTOCOMPLETE_MENU)
       end
@@ -273,6 +287,11 @@ module PageObjects
         select_kit.collapse
       end
 
+      def toggle_rich_editor
+        find("#{COMPOSER_ID} .composer-toggle-switch").click
+        self
+      end
+
       private
 
       def emoji_preview_selector(emoji)