DEV: Compile markdown-it-bundle with ember-cli (#18104)

We were already compiling the markdown bundle via ember-cli, but that version was only being used in the test environment. This commit improves the implementation, and updates the filename so it's also used in production.

This commit also
- Removes the vendored copy of `markdown-it.js` and fetches from node_modules instead
- Updates `pretty_text.rb` to remove the custom sprockets-manifest-parsing
- Removes `pretty-text-bundle.js`, which was only being used by `pretty_text.rb`

lib/tasks/javascript.rake

@@ -89,8 +89,6 @@ def dependencies
     }, {
       source: '@highlightjs/cdn-assets/.',
       destination: 'highlightjs'
-    }, {
-      source: 'markdown-it/dist/markdown-it.js'
     }, {
       source: 'moment/moment.js'
     }, {