LibJS: Implement basic support for the "new" keyword

NewExpression mostly piggybacks on the existing CallExpression. The big
difference is that "new" creates a new Object and passes it as |this|
to the callee.

Libraries/LibJS/Parser.h

@@ -63,6 +63,7 @@ public:
     NonnullRefPtr<ArrayExpression> parse_array_expression();
     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();
 
     bool has_errors() const { return m_has_errors; }