Fix: no-unused-vars false positive around callback (fixes #6576)

[mysticatea]

Fixes #6576.

I added a check to check whether the Identifier of a read reference exists inside of a function node which can be used later or not.

"can be used later" is meant:

• the function is assigned to a variable.
• the function is bound to a property, and the object can be used later.
• the function is bound to an argument of a function call.

If a reference exists in a function which can be used later, the reference is read when the function is called.

[eslintbot]

LGTM

[eslintbot]

LGTM

[mysticatea]

I added a check to cover #6576 (comment) in this PR.
In that case, the reference itself can be used later.

[nzakas]

LGTM. Can you update the docs to account for what "can be used later" means?

[nzakas]

I'll update the docs so we can merge this.

[nzakas]

Actually, I'm going to hold off on merging this for now as I have some questions back on the issue.

[nzakas]

Okay, I misunderstood something. Nevermind. :)

[eslintbot]

LGTM