Add -s flag to //stack worldedit command

[max1220]

Add -s flag to //stack WorldEdit command to shift the selection to the target location.

This makes repeated stacking way more convenient, and further increases WorldEdit parity.

I've done some minimal testing, and this behaved as expected. The implementation is almost exactly what -s does in the //move command.

Any feedback welcome!

[StackDoubleFlow]

I didn't even realize //stack had -s

It looks like the real Worldedit help entry for //stack describes the -s flag as "Shift the selection to the last stacked copy". I think that's more clear in this case, so could you change the description for -s to be the same?

Also, please run cargo fmt for formatting. Thanks!