Update Action Text's Trix dependency

There is a new [Trix.js][] release, bumping the version from `1.3.1` to
`2.0.4`.

Applications that consume Action Text through the `@rails/actiontext`
npm package can manage their own Trix dependency version.

Similarly, Importmaps users can `pin "trix", to: "..."` to a CDN serving
`2.0.4`.

For the sake of applications that depend on Trix through the
`action_text` gem, this commit updates the pre-bundled JS and CSS code.

These changes were generated by executing the following:

```sh
cd actiontext
yarn build
cp ../node_modules/trix/dist/trix.css app/assets/stylesheets/trix.css
cp ../node_modules/trix/dist/trix.esm.js app/assets/javascripts/trix.js
```

[Trix.js]: https://trix-editor.org
[v2.0.4]: https://github.com/basecamp/trix/releases/tag/v2.0.4

actiontext/CHANGELOG.md

@@ -1,3 +1,7 @@
+*   Update bundled Trix version from `1.3.1` to `2.0.4`.
+
+    *Sean Doyle*
+
 *   Apply `field_error_proc` to `rich_text_area` form fields.
 
     *Kaíque Kandy Koga*