Add doppler secrets edit command

This works pretty well from my testing, but there are more edge cases to handle before this is ready to merge. At a minimum, this needs:

[ ] Tests around the parsing logic
[ ] Tests around edge cases (like saving an empty file or a file that we can't parse)
[ ] Delete secret support? (compare against the ones you chose, delete any not present in the edited version)
[ ] Better copy and help text
[ ] Research if Windows has a way to edit a file that's in memory and not on disk

Demo workflow:

DopplerHQ / cli