DEV: Introduce a value transformer front-end plugin API (#27090)

This commit introduces the `valueTransformer`API to safely override values defined in Discourse.

Two new plugin APIs are introduced:

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

It also introduces the function `applyValueTransformer` which can be imported from `discourse/lib/transformer`. This function marks the desired value as overridable and applies the transformer logic.

How does it work?

## Marking a value as overridable:
 
To mark a value 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 `addValueTransformerName` instead.

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

- example:

```js
export default class HomeLogo extends Component {
  @service session;
  @service site;
  ...
  get href() {
    return applyValueTransformer("home-logo-href", getURL("/"));
  }	
```

## Overriding a value in plugins or themes

To override a value in plugins, themes, or TCs use the plugin API `registerValueTransformer`:

- Example:

```js
withPluginApi("1.34.0", (api) => {
  api.registerValueTransformer("example-transformer", ({ value }) => {
    return "new-value";
  });
});
```

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).
 
+## [1.34.0] - 2024-06-06
+
+- Added `registerValueTransformer` which allows registering a transformer callback to override values defined in Discourse modules
+- Added `addValueTransformerName` which allows plugins/TCs to register a new transformer to override values defined in their modules
+
 ## [1.33.0] - 2024-06-06
 
 - Added `addCustomUserFieldValidationCallback` which allows to set a callback to change the validation and user facing message when attempting to save the signup form.