Add String.replacing(:maxReplacements:with:) with a closure

[dfabulich]

#expect("1zz1" == "1z1".replacing(try Regex("([a-z])"), with: { match in
    return match[1].substring! + match[1].substring!
}))

This lets you replace a matching groups with dynamic content.

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I wrote the test; Cursor wrote the code. (It's not a lot of code.) The test fails on the main branch, and passes here.