Decorator parsing is too forgiving

[adams85]

According to the proposal, when decorator expression is not wrapped in parentheses, it must be an identifier or a special member expression (which can contain . member access only). Other expressions must be parenthesized. See also https://github.com/tc39/proposal-decorators#syntax

Currently we just parse it as a plain LHS expression, so invalid syntaxes like @d['a'], @d?.a, @function(){ } class X {} are accepted.

AST to JS formatter doesn't deal with this correctly either, must be adjusted too.