bug #5928 fix(trix): fix trix editor config loading (welcoMattic)

This PR was merged into the 4.x branch.

Discussion
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fix(trix): fix trix editor config loading

Ref #5725

This PR change the listen event to boot TextEditorField. Before it listen on `DOMLoadedContent`. Now it listen on `trix-before-initialize` to be sure to handle the custom configuration and render properly a `heading1` element with a custom tagName `h2` for example.

If needed, I can add some tests to cover it.

Commits
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1f49189 fix(trix): fix trix editor config loading

assets/js/field-text-editor.js

@@ -6,7 +6,9 @@ import Trix from 'trix/dist/trix';
 // Provide Trix variable globally to allow custom backend pages to use it
 window.Trix = Trix;
 
-document.addEventListener('DOMContentLoaded', () => {
+// Listening to the DOMLoadedContent event is too late because the Trix editor is already initialized.
+// To be sure to handle properly the custom configuration, we have to to listen to the trix-before-initialize event.
+document.addEventListener('trix-before-initialize', () => {
     new TextEditorField();
 });