patch 8.2.4257: Vim9: finding global function without g: prefix inconsistent

Problem:    Vim9: finding global function without g: prefix but not finding
            global variable is inconsistent.
Solution:   Require using g: for a global function.  Change the vim9.vim
            script into a Vim9 script with exports.  Fix that import in legacy
            script does not work.
diff --git a/src/userfunc.c b/src/userfunc.c
index 8c20439..097d4f1 100644
--- a/src/userfunc.c
+++ b/src/userfunc.c
@@ -3789,7 +3789,7 @@
 	    sid_buf[1] = KS_EXTRA;
 	    sid_buf[2] = (int)KE_SNR;
 	    vim_snprintf((char *)sid_buf + 3, sizeof(sid_buf) - 3,
-					    "%ld_", (long)current_sctx.sc_sid);
+						      "%ld_", (long)lv.ll_sid);
 	    name = concat_str(sid_buf, lv.ll_name);
 	}
 	*lv.ll_name_end = cc;