Add prop argument to rep_slice_sample()

[ismayc]

It's useful when bootstrapping to specify a prop argument instead of the number of rows if keeping things basic and not getting into the {infer} pipeline. This was requested on the {moderndive} package here, but since we import rep_sample_n() from {infer} we probably should import rep_slice_sample() as well. We'd prefer to not have two different versions of the same function floating around (one with prop and one without).

[simonpcouch]

All for it!🦥 Will put in a PR here in a bit.

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