Expose hasInitializer publicly

[ayazhafiz]

Exposes hasInitializer as a public function so that users of
TypeScript APIs do not have to roll their own HasInitializer type
guards.

Closes #33115.

[msftclas]

All CLA requirements met.

[weswigham]

Our internal hasInitializer "guard" isn't really safe guard - it (just) checks if an initializer field is actually present, because in many cases it is optional (and so using it as a type guard in incorrect in the false branch case) - the implementation using node kinds which you had in your issue is likely much more correct for your usecase.

[ayazhafiz]

@weswigham would you prefer creating a new guard or changing the existing one?

[weswigham]

A new one, probably

[ayazhafiz]

Closing in favor of #33229