Update the spec parser to take stop and continuation tokens into account for `<typeArguments>` as a selector

[eernstg]

As described in https://github.com/dart-lang/language/blob/main/accepted/2.15/constructor-tearoffs/feature-specification.md#grammar-ambiguities, the derivation from <selector> to <typeArguments> can only be applied when the lookahead is a stop token or a continuation token, as defined in said section.

Currently, Dart.g allows <selector> to derive <typeArguments> no matter which token occurs next. This issue serves as a request to implement the missing check.