patch 8.2.1637: Vim9: :put ={expr} does not work inside :def function

Problem:    Vim9: :put ={expr} does not work inside :def function.
Solution:   Add ISN_PUT. (closes #6397)
diff --git a/src/mouse.c b/src/mouse.c
index 7af9049..7b1bedc 100644
--- a/src/mouse.c
+++ b/src/mouse.c
@@ -430,7 +430,8 @@
 		    insert_reg(regname, TRUE);
 		else
 		{
-		    do_put(regname, BACKWARD, 1L, fixindent | PUT_CURSEND);
+		    do_put(regname, NULL, BACKWARD, 1L,
+						      fixindent | PUT_CURSEND);
 
 		    // Repeat it with CTRL-R CTRL-O r or CTRL-R CTRL-P r
 		    AppendCharToRedobuff(Ctrl_R);
@@ -849,7 +850,7 @@
 	// to this position
 	if (restart_edit != 0)
 	    where_paste_started = curwin->w_cursor;
-	do_put(regname, dir, count, fixindent | PUT_CURSEND);
+	do_put(regname, NULL, dir, count, fixindent | PUT_CURSEND);
     }
 
 #if defined(FEAT_QUICKFIX)