This change is motivated by user feedback where we saw some confusion
where they didn't know that you could create either a worktree or branch
through simply typing in their respective picker's editor. Hopefully,
this new placeholder in each of them will help communicate that.

Release Notes:

- Improved placeholder copywriting in both the branch and worktree
pickers to communicate it's possible to type-to-create each.

Co-authored-by: María Craig <maria@zed.dev>