add firefox extension template

externs/webextension_polyfill/bookmarks/BookmarkTreeNodeUnmodifiable.hx

@@ -0,0 +1,8 @@
+package webextension_polyfill.bookmarks;
+
+/**
+	Indicates the reason why this node is unmodifiable. The <var>managed</var> value indicates that this node was configured
+	by the system administrator or by the custodian of a supervised user. Omitted if the node can be modified by the user
+	and the extension (default).
+**/
+typedef BookmarkTreeNodeUnmodifiable = String;