This change adds support for determining the editor that should be opened by the RAD_EDITOR environment variable, which takes priority over both VISUAL and EDITOR, as it is specific to the radicle application.

This enables users to set a different editor specifically for radicle, which is useful in the event that the normal value for either VISUAL or EDITOR is a process that forks and exits early, but supports command line flags to modify this behavior and block on the main thread (e.g. neovim-remote, which the author uses).