completions: Add "arrow-head" as possible value for isNewIdentifierLocation

[ghost]

Fixes #25691
@amcasey Is there any chance that receiving a non-boolean here would break Visual Studio?

[RyanCavanaugh]

I think this is quite likely to break VS

[amcasey]

I suppose there's a tiny chance that Newtonsoft silently drops the bad value. But I would also expect it to break.

[amcasey]

Nope, looks like you'd get no completion when a non-Boolean was returned.

[ghost]

Maybe we should create a new field that's always a string and deprecate the old one?

[ghost]

@amcasey @mhegazy Opinions on how to expose this?

[amcasey]

Adding a new field seems reasonable. A couple questions:

1. Old versions of VS are going to continue to consume the old field. Do I understand correctly that the old behavior basically treats arrow-head as true and that we'll continue to set it that way in the future?
2. I don't think "arrow-head" clearly conveys the meaning given in the jsdoc comment. Is it even important to callers that the special case is for arrow functions? Would it be clearer to describe the context (e.g. declaration (currently, true), reference (current, false), expression (i.e. unknown))?

[RyanCavanaugh]

Just adding a ❌ here so we don't accidently merge this before we've decided what to do