DEV: Add behavior transformers (#27409)

This commit introduces the `behaviorTransformer` API to safely override behaviors defined in Discourse.

Two new plugin APIs are introduced:

- `addBehaviorTransformerName` which allows plugins and theme-components to add a new valid transformer name if they want to provide overridable behaviors;
- `registerBehaviorTransformer` to register a transformer to override behaviors.

It also introduces the function `applyBehaviorTransformer` which can be imported from `discourse/lib/transformer`. This is used to mark a callback containing the desired behavior as overridable and applies the transformer logic.

How does it work?

## Marking a behavior as overridable:
 
To mark a behavior as overridable, in Discourse core, first the transformer name must be added to `app/assets/javascripts/discourse/app/lib/transformer/registry.js`. For plugins and theme-components, use the plugin API `addBehaviorTransformerName` instead.

Then, in your component or class, use the function `applyBehaviorTransformer` to mark the Behavior as overridable and handle the logic:

- example:

```js
  ...
  @action
  loadMore() {
    applyBehaviorTransformer(
      "discovery-topic-list-load-more",
      () => {
        this.documentTitle.updateContextCount(0);
        return this.model
          .loadMore()
          .then(({ moreTopicsUrl, newTopics } = {}) => {
            if (
              newTopics &&
              newTopics.length &&
              this.bulkSelectHelper?.bulkSelectEnabled
            ) {
              this.bulkSelectHelper.addTopics(newTopics);
            }
            if (moreTopicsUrl && $(window).height() >= $(document).height()) {
              this.send("loadMore");
            }
          });
      },
      { model: this.model }
    );
  },
  ...	
```

## Overriding a behavior in plugins or themes

To override a behavior in plugins, themes, or TCs use the plugin API `registerBehaviorTransformer`:

- Example:

```js
withPluginApi("1.35.0", (api) => {
  api.registerBehaviorTransformer("example-transformer", ({ context, next }) => {
    console.log('we can introduce new behavior here instead', context);

    next(); // call next to execute the expected behavior
  });
});
```

docs/CHANGELOG-JAVASCRIPT-PLUGIN-API.md

@@ -7,6 +7,11 @@ in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [1.35.0] - 2024-07-30
+
+- Added `registerBehaviorTransformer` which allows registering a transformer callback to override behavior defined in Discourse modules
+- Added `addBehaviorTransformerName` which allows plugins/TCs to register a new transformer to override behavior defined in their modules
+
 ## [1.34.0] - 2024-06-06
 
 - Added `registerValueTransformer` which allows registering a transformer callback to override values defined in Discourse modules