patch 8.2.4723: the ModeChanged autocmd event is inefficient

Problem:    The ModeChanged autocmd event is inefficient.
Solution:   Avoid allocating memory. (closes #10134)  Rename
            trigger_modechanged() to may_trigger_modechanged().
diff --git a/src/terminal.c b/src/terminal.c
index 0849f2c..b6efb49 100644
--- a/src/terminal.c
+++ b/src/terminal.c
@@ -2035,7 +2035,7 @@
 set_terminal_mode(term_T *term, int normal_mode)
 {
     term->tl_normal_mode = normal_mode;
-    trigger_modechanged();
+    may_trigger_modechanged();
     if (!normal_mode)
 	handle_postponed_scrollback(term);
     VIM_CLEAR(term->tl_status_text);