Add `cabal gen-bounds` write-in-place option

[emilypi]

gen-bounds is a great feature. It should have a write-to-cabal-in-place option. This is a small, but impactful quality of life improvement to the gen-bounds command. Just a spitball approach would be to add a flag for --write-in-place or --write, and to modify the existing .cabal file dependencies with bounds.