LibJS: Add support for arrow functions

Author: https://github.com/jack-karamanian
Commit: 098f1cd0ca
Pull-request: https://github.com/SerenityOS/serenity/pull/1532
Reviewed-by: https://github.com/awesomekling

Libraries/LibJS/Parser.h

@@ -67,6 +67,7 @@ public:
     NonnullRefPtr<Expression> parse_secondary_expression(NonnullRefPtr<Expression>, int min_precedence, Associativity associate = Associativity::Right);
     NonnullRefPtr<CallExpression> parse_call_expression(NonnullRefPtr<Expression>);
     NonnullRefPtr<NewExpression> parse_new_expression();
+    RefPtr<FunctionExpression> try_parse_arrow_function_expression(bool expect_parens);
 
     bool has_errors() const { return m_parser_state.m_has_errors; }