LibWeb: Stub out a few HTMLTextArea validity IDL methods

These are invoked by GitHub when submitting a comment. Stub them out for
now, as this is enough to let GitHub proceed with (attempting) to submit
the comment.

Userland/Libraries/LibWeb/HTML/HTMLTextAreaElement.h

@@ -79,6 +79,10 @@ public:
 
     u32 text_length() const;
 
+    bool check_validity();
+    bool report_validity();
+    void set_custom_validity(String const& error);
+
     WebIDL::Long max_length() const;
     WebIDL::ExceptionOr<void> set_max_length(WebIDL::Long);