chat: closing reply preview with action (#7517)

This is a follow up to #7170. Closing the reply to preview overlay is
now configurable with an action (keybinding set escape in the default
keymap).


https://github.com/zed-industries/zed/assets/53836821/d679e734-f90b-4490-8e79-7dfe5407988a


Release Notes:
- N/A