DEV: Convert composer controller to native class syntax (#20723)

Actions are moved from actions: {} to top-level functions with @action decorator. Previously we had a save() action and a top-level function of the same name, so this commit renames the action to avoid a clash.

app/assets/javascripts/discourse/app/templates/composer.hbs

@@ -4,7 +4,7 @@
   @openIfDraft={{action "openIfDraft"}}
   @typed={{action "typed"}}
   @cancelled={{action "cancelled"}}
-  @save={{action "save"}}
+  @save={{this.saveAction}}
 >
   <div class="grippie"></div>
 
@@ -234,7 +234,7 @@
 
           <div class="save-or-cancel">
             <ComposerSaveButton
-              @action={{action "save"}}
+              @action={{this.saveAction}}
               @icon={{this.saveIcon}}
               @label={{this.saveLabel}}
               @forwardEvent={{true}}