proposal: slices: add Map function

[haoxins]

It would be nice to have a simple, generic way to map a slice to another.
I have already seen it in the associated drafts, such as https://github.com/golang/proposal/blob/master/design/go2draft-contracts.md#defined-aggregate-types,
but it is not included in the slices package introduced in Go 1.21.

Is there any particular reason not to do this?

[seankhliao]

Duplicate of #58881