patch 8.2.1342: Vim9: accidentally using "t" gives a confusing error

Problem:    Vim9: accidentally using "x" gives a confusing error.
Solution:   Disallow using ":t" in Vim9 script. (issue #6399)
diff --git a/src/vim9compile.c b/src/vim9compile.c
index 97fa2e0..38c097c 100644
--- a/src/vim9compile.c
+++ b/src/vim9compile.c
@@ -7467,6 +7467,7 @@
 	    case CMD_append:
 	    case CMD_change:
 	    case CMD_insert:
+	    case CMD_t:
 	    case CMD_xit:
 		    not_in_vim9(&ea);
 		    goto erret;