patch 8.2.2658: :for cannot loop over a string

Problem:    :for cannot loop over a string.
Solution:   Accept a string argument and iterate over its characters.
diff --git a/src/vim9compile.c b/src/vim9compile.c
index c5f0f2a..c1adb36 100644
--- a/src/vim9compile.c
+++ b/src/vim9compile.c
@@ -7264,11 +7264,15 @@
     }
     arg_end = arg;
 
-    // Now that we know the type of "var", check that it is a list, now or at
-    // runtime.
+    // If we know the type of "var" and it is a not a list or string we can
+    // give an error now.
     vartype = ((type_T **)stack->ga_data)[stack->ga_len - 1];
-    if (need_type(vartype, &t_list_any, -1, 0, cctx, FALSE, FALSE) == FAIL)
+    if (vartype->tt_type != VAR_LIST && vartype->tt_type != VAR_STRING
+						&& vartype->tt_type != VAR_ANY)
     {
+	// TODO: support Blob
+	semsg(_(e_for_loop_on_str_not_supported),
+					       vartype_name(vartype->tt_type));
 	drop_scope(cctx);
 	return NULL;
     }