Accept union replacement values in String.replace

[Copilot]

Description

String.prototype.replace accepts either a string or callback replacement, but its overloads rejected variables typed as their union.

• Replace separate overloads with a union-typed replacement argument.
• Apply the same contract to custom [Symbol.replace] search objects.
• Add a compiler regression test for both paths.

declare const replacement: string | ((match: string) => string);

"text".replace(/./, replacement);

• Fixes Notes about String.replace #54387