LibJS: Add function default arguments

Adds the ability for function arguments to have default values. This
works for standard functions as well as arrow functions. Default values
are not printed in a <function>.toString() call, as nodes cannot print
their source string representation.

Libraries/LibJS/Parser.h

@@ -108,6 +108,6 @@ private:
     };
 
     ParserState m_parser_state;
-    Optional<ParserState> m_saved_state;
+    Vector<ParserState> m_saved_state;
 };
 }