[JavaScript] Rationalize function name detection.

[Thom1729]

In some cases, the current JavaScript syntax will highlight a variable name, mapping key, or property name as a function name (entity.name.function):

const f = function() {};

const obj = {
    f: function() {}
};

obj.f = function() {};

Originally, this seems to have been a convenience feature for display purposes. The implementation uses lookaheads and is not reliable, leading to an uneven user experience.

However, there is a real JavaScript feature by which anonymous functions will be named using the surrounding context in some cases. We could detect most of these cases using branching and reimplement the special name highlighting in terms of the language feature. This would seem to be the most correct way of handling what is otherwise a vaguely-motivated special case.

[michaelblyons]

Closed with #2417?

[Thom1729]

That one improves arrow function detection, but it still uses awkward lookaheads to highlight the “names” of anonymous functions. I've been looking into ways to rewrite that to avoid the messy lookaheads, but haven't hit upon the right implementation yet.