Extract `ReplaceDeep` function into a separate module

[ahrjarrett]

After merging

While implementing https://github.com/sindresorhus/type-fest/pull/648 @sindresorhus and I discussed extracting ReplaceDeep into its own type. This issue reflects the work to do that.

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[ahrjarrett]

Sindre, if you'd like to assign this issue to me, that will put it on my todo list so I don't forget.