DEV: Support @debounce decorator in native class syntax (#20521)

The implementation previously generated a descriptor with an `initializer()`, and bound the function to the `this` context of the initializer. In native class syntax, the initializer of a descriptor is only called once, with a `this` context of the constructor, not the instance.

This commit updates the implementation so that it generates the bound function on-demand using a getter. This is the same strategy employed by ember's built-in `@action` decorator.

Unfortunately, this use of a getter means that the `@observes` decorator does not support being directly chained to `@debounce`. It throws the error "`observer must be provided a function or an observer definition`". The workaround is to put the observer on its own function, which then calls the debounced function. Given that we're aiming to reduce our usage of `@observes`, we've accepted the need for this workaround rather than spending the time to patch the implementation of `@observes`.

plugins/discourse-local-dates/assets/javascripts/discourse/components/discourse-local-dates-create-form.js

@@ -61,6 +61,10 @@ export default Component.extend({
   },
 
   @observes("computedConfig.{from,to,options}", "options", "isValid", "isRange")
+  configChanged() {
+    this._renderPreview();
+  },
+
   @debounce(INPUT_DELAY)
   async _renderPreview() {
     if (this.markup) {