DEV: Switch to new addComposerToolbarPopupMenuOption plugin API (#65)

Why this change?

`api.addToolbarPopupMenuOptionsCallback` has been deprecated in https://github.com/discourse/discourse/commit/913fd3a7b392b492f6344102577960a6eada00ce

javascripts/discourse/initializers/disco-toc-composer.js

@@ -19,27 +19,19 @@ export default {
         }
         I18n.translations[I18n.currentLocale()].js.composer.contains_dtoc = " ";
 
-        api.modifyClass("controller:composer", {
-          pluginId: "DiscoTOC",
-
-          actions: {
-            insertDtoc() {
-              this.get("toolbarEvent").applySurround(
-                `<div data-theme-toc="true">`,
-                `</div>`,
-                "contains_dtoc"
-              );
-            },
+        api.addComposerToolbarPopupMenuOption({
+          action: (toolbarEvent) => {
+            toolbarEvent.applySurround(
+              `<div data-theme-toc="true">`,
+              `</div>`,
+              "contains_dtoc"
+            );
+          },
+          icon: "align-left",
+          label: themePrefix("insert_table_of_contents"),
+          condition: (composer) => {
+            return composer.model.topicFirstPost;
           },
-        });
-
-        api.addToolbarPopupMenuOptionsCallback((controller) => {
-          return {
-            action: "insertDtoc",
-            icon: "align-left",
-            label: themePrefix("insert_table_of_contents"),
-            condition: controller.get("model.topicFirstPost"),
-          };
         });
       }
     });