:stopinsert should leave terminal-mode #9856

Problem:  Calling :stopinsert from RPC while in terminal-mode does not
          go back to normal-mode.
Solution: Implement a check() handler for state_enter(), adapted from
          insert_check().

Fix #7807

src/nvim/terminal.c

@@ -403,6 +403,7 @@ void terminal_enter(void)
   redraw(false);
 
   s->state.execute = terminal_execute;
+  s->state.check = terminal_check;
   state_enter(&s->state);
 
   restart_edit = 0;
@@ -427,6 +428,19 @@ void terminal_enter(void)
   }
 }
 
+// Function executed before each iteration of terminal mode.
+// Return:
+//   1 if the iteration should continue normally
+//   0 if the main loop must exit
+static int terminal_check(VimState *state)
+{
+  if (stop_insert_mode) {
+    stop_insert_mode = false;
+    return 0;
+  }
+  return 1;
+}
+
 static int terminal_execute(VimState *state, int key)
 {
   TerminalState *s = (TerminalState *)state;