ui: Reimplement :suspend command for remote UIs.

- Remove suspend method from the UI protocol
- Handle `:suspend` by disconnecting the last channel that sent a request to
  nvim.

src/nvim/globals.h

@@ -1245,6 +1245,9 @@ EXTERN int curr_tmode INIT(= TMODE_COOK); /* contains current terminal mode */
 EXTERN bool embedded_mode INIT(= false);
 // Using the "abstract_ui" termcap
 EXTERN bool abstract_ui INIT(= false);
+// Id of the last channel sent a message to nvim. Used to determine the target
+// of channel-specific actions such as suspending
+EXTERN uint64_t last_message_source INIT(= 0);
 
 /// Used to track the status of external functions.
 /// Currently only used for iconv().