feat(terminal): implement <c-\><c-o> for terminal mode

this works similar to <c-o> or <c-\><c-o> in insert mode

src/nvim/edit.c

@@ -1223,10 +1223,10 @@ bool edit(int cmdchar, bool startln, long count)
       // the value of `restart_edit` before `ex_normal` returns.
       restart_edit = 'i';
       force_restart_edit = true;
+      return false;
     } else {
-      terminal_enter();
+      return terminal_enter();
     }
-    return false;
   }
 
   // Don't allow inserting in the sandbox.