term/editor: add support for RAD_EDITOR — heartwood

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).

         let Some(cmd) = self::default_editor() else {
             return Err(io::Error::new(
                 io::ErrorKind::NotFound,
                 "editor not configured: the `EDITOR` environment variable is not set",
                 "editor not configured: please set `RAD_EDITOR`, `VISUAL`, or `EDITOR` to resolve",
             ));
         };

 /// Get the default editor command.
 fn default_editor() -> Option<OsString> {
     // First check the standard environment variables.
     if let Ok(rad_editor) = env::var("RAD_EDITOR") {
         if !rad_editor.is_empty() {
             return Some(rad_editor.into());
         }
     }
     if let Ok(visual) = env::var("VISUAL") {
         if !visual.is_empty() {
             return Some(visual.into());