[IMP] web_editor: allow onRemove to be async

Now, a snippet option can do async operations in its onRemove
implementation.

task-2636545

X-original-commit: ef2e55d7f407dedfbf724f2cae32a0fcaede6890
Part-of: odoo/odoo#77256

addons/web_editor/static/src/js/editor/snippets.editor.js

@@ -451,9 +451,9 @@ var SnippetEditor = Widget.extend({
         await new Promise(resolve => {
             this.trigger_up('call_for_each_child_snippet', {
                 $snippet: this.$target,
-                callback: function (editor, $snippet) {
+                callback: async function (editor, $snippet) {
                     for (var i in editor.styles) {
-                        editor.styles[i].onRemove();
+                        await editor.styles[i].onRemove();
                     }
                 },
                 onSuccess: resolve,

addons/web_editor/static/src/js/editor/snippets.options.js

@@ -2950,8 +2950,9 @@ const SnippetOptionWidget = Widget.extend({
      * Called when the associated snippet is about to be removed from the DOM.
      *
      * @abstract
+     * @returns {Promise|undefined}
      */
-    onRemove: function () {},
+    onRemove: async function () {},
     /**
      * Called when the target is shown, only meaningful if the target was hidden
      * at some point (typically used for 'invisible' snippets).