Allow Infinity, -Infinity and NaN as an enum key name for string enum about typescript

[MartinJohns]

This was allowed before, but then it was intentionally removed again: #56161

[RyanCavanaugh]

I don't think this rule makes sense to apply to string enums. Will discuss.

[DanielRosenwasser]

It's not purely about string enum members. It depends entirely on whether other members contribute a reverse mapping that could conflict:

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enum E {
    Infinity = "boop",
    blah = 1/0,
}

console.log(E.Infinity); // prints "blah", not "boop"!

I other words, we need to keep the keys and reverse-mapping keys disjoint so one doesn't stomp on the other.

There's basically two ways of relaxing this:

1. numerically named string enum members are allowed if all members of an enum are string-based
2. numerically named string enum members are allowed if no other enum member contributes a conflicting reverse mapping

(1) is just a heavier restriction of (2) because it'll be vacuously true - there will never be any conflicting reverse mappings because there won't be any to begin with on an all-string enum.