patch 8.2.3503: Vim9: using g:pat:cmd is confusing

Problem:    Vim9: using g:pat:cmd is confusing.
Solution:   Do not recognize g: as the :global command.  Also for s:pat:repl.
            (closes #8982)

src/ex_cmds.c

@@ -3724,6 +3724,9 @@ ex_substitute(exarg_T *eap)
 				// don't accept alphanumeric for separator
 	if (check_regexp_delim(*cmd) == FAIL)
 	    return;
+	if (in_vim9script() && check_global_and_subst(eap->cmd, eap->arg)
+								      == FAIL)
+	    return;
 
 	/*
 	 * undocumented vi feature:
@@ -4899,6 +4902,9 @@ ex_global(exarg_T *eap)
     cmd = eap->arg;
     which_pat = RE_LAST;	    // default: use last used regexp
 
+    if (in_vim9script() && check_global_and_subst(eap->cmd, eap->arg) == FAIL)
+	return;
+
     /*
      * undocumented vi feature:
      *	"\/" and "\?": use previous search pattern.