Re-add strict concat signature

[sandersn]

Fixes #9901. Broken by #6629 after we missed the bug during the discussion in #6594.
#6629 introduces a new type for concat: (...items: (T | T[])[]): T[]. This means that concat accepts either list-wrapped arguments (T[]) or top-level arguments (T):

numbers.concat(1, [2, 3], 4)

Here, (1) and (4) are top-level arguments (T) whereas (2) and (3) are wrapped-arguments. Unfortunately, when you have a tuple or list type parameter, it's ambiguous whether an argument is list-wrapped or top-level:

pairs.concat([[1, 2], [3, 4]]);

If [[1,2], [3, 4]] is a top-level argument, then its type is number[][] (or [number, number][] or [[number, number], [number, number]]). If it's a list-wrapped argument, then its type is number[] (or [number, number]). But TypeScript can't tell, so it has to guess. Unfortunately, it guesses "top-level argument" (T) with type number[][]/[number, number][]. In the #9901 repro, number[][] is not compatible with the type argument T=[number, number].

Adding a stricter overload first guides the inference process to prefer the list-wrapped-only case (T[] only) before falling back to the mixed top-level/list-wrapped overload.

[weswigham]

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