Make Action Text + Trix JS/CSS available via the asset pipeline (#42857)

* Action Text JS should be available via the asset pipeline too

* Main was a module anyway, no need to reference that twice

* Fix rollup references

* Precompile action text JS for asset pipeline

* No JavaScript dependencies needed with the asset pipeline

* Stub Webpacker::Engine to trigger webpack path for testing

* Extract asset paths

* Exercise asset pipeline path

* Terser doesn't do anything useful on this small package

* Make trix directly available to the asset pipeline

* Indirect doesn't carry its worth

* Reminder for development about keeping things in sync for the asset pipeline

* Ensure this isn't turned into undefined while mirroring

* Mirror Trix CSS for asset pipeline

* Add the needed JS include tag automatically under the asset pipeline

* Please RuboCop

* Keep the peer dependency

Even though we also need it explicitly as a dev dependency in order to generate the mirror output for trix.

* Fix test

* Add CHANGELOG entry

actiontext/CHANGELOG.md

@@ -1,3 +1,7 @@
+*   Make the Action Text + Trix JavaScript and CSS available through the asset pipeline.
+
+    *DHH*
+
 *   OpenSSL constants are now used for Digest computations.
 
     *Dirkjan Bussink*

actiontext/README.md

@@ -4,6 +4,10 @@ Action Text brings rich text content and editing to Rails. It includes the [Trix
 
 You can read more about Action Text in the [Action Text Overview](https://edgeguides.rubyonrails.org/action_text_overview.html) guide.
 
+## Development
+
+The JavaScript for Action Text is distributed both as a npm module under @rails/actiontext and via the asset pipeline as action_text.js (and we mirror Trix as trix.js). To ensure that the latter remains in sync, you must run `yarn build` and checkin the artifacts whenever the JavaScript source or the Trix dependency is bumped. CSS changes must be brought over manually to app/assets/stylesheets/trix.css
+
 ## License
 
 Action Text is released under the [MIT License](https://opensource.org/licenses/MIT).