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)

src/vim9compile.c

@@ -7467,6 +7467,7 @@ compile_def_function(ufunc_T *ufunc, int set_return_type, cctx_T *outer_cctx)
 	    case CMD_append:
 	    case CMD_change:
 	    case CMD_insert:
+	    case CMD_t:
 	    case CMD_xit:
 		    not_in_vim9(&ea);
 		    goto erret;