Expose hasOnlyExpressionInitializer as a public type guard

[ayazhafiz]

Exposes hasOnlyExpressionInitializer as a public function so users of
TypeScript compiler APIs do not have to roll their own
HasExpressionInitializer type guards.

Closes #33115.

[ayazhafiz]

Does the TypeScript project have automated code formatter or formatting configuration other than the wiki coding guidelines?

[Jessidhia]

(this is not this PR's fault but) the isHasExpressionInitializer name is highly confusing. What is a "Has expression"? 🙃

[sheetalkamat]

I don’t think we want to add new api/utility function that we ourselves are not using ..,

[ayazhafiz]

(this is not this PR's fault but) the isHasExpressionInitializer name is highly confusing. What is a "Has expression"? 🙃

I agree, the name is rather confusing especially if the HasExpressionInitializer type is not already known about. I've exposed hasOnlyExpressionInitializer instead, which should make more sense.

I don’t think we want to add new api/utility function that we ourselves are not using ..,

Fixed by exposing hasOnlyExpressionInitializer publicly and modifying it to be a true type-safe guard (checking node types rather than the existence of a initializer field). Please let me know if this is an unsuitable solution.

[sandersn]

The code change itself seems correct and likely to be faster than checking for .initializer and then excluding things.