LibJS: Implement 'new.target'

This adds a new MetaProperty AST node which will be used for
'new.target' and 'import.meta' meta properties. The parser now
distinguishes between "in function context" and "in arrow function
context" (which is required for this).
When encountering TokenType::New we will attempt to parse it as meta
property and resort to regular new expression parsing if that fails,
much like the parsing of labelled statements.

Libraries/LibJS/Runtime/ScriptFunction.cpp

@@ -105,6 +105,8 @@ LexicalEnvironment* ScriptFunction::create_environment()
     auto* environment = heap().allocate<LexicalEnvironment>(global_object(), move(variables), m_parent_environment, LexicalEnvironment::EnvironmentRecordType::Function);
     environment->set_home_object(home_object());
     environment->set_current_function(*this);
+    if (m_is_arrow_function)
+        environment->set_new_target(m_parent_environment->new_target());
     return environment;
 }