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>

script/build_jsconfig.rb

@@ -31,6 +31,7 @@ def write_config(package_dir, extras: {})
       "module" => "esnext",
       "moduleResolution" => "bundler",
       "experimentalDecorators" => true,
+      "allowJs" => true,
       "paths" => {
         **namespaces
           .map { |ns, paths| [ns, paths.map { |p| "#{relative(package_dir, p)}/*" }] }