patch 8.2.3347: check for legacy script is incomplete

Problem:    Check for legacy script is incomplete. (Naohiro Ono)
Solution:   Also check the :legacy modifier.  Use for string concatenation
            with "." and others (issue #8756)
diff --git a/src/vim9script.c b/src/vim9script.c
index 66305a3..1999109 100644
--- a/src/vim9script.c
+++ b/src/vim9script.c
@@ -34,6 +34,18 @@
 
 #if defined(FEAT_EVAL) || defined(PROTO)
 /*
+ * Return TRUE when currently in a script with script version smaller than
+ * "max_version" or command modifiers forced it.
+ */
+    int
+in_old_script(int max_version)
+{
+    return (current_sctx.sc_version <= max_version
+					 && !(cmdmod.cmod_flags & CMOD_VIM9CMD))
+		|| (cmdmod.cmod_flags & CMOD_LEGACY);
+}
+
+/*
  * Return TRUE if the current script is Vim9 script.
  * This also returns TRUE in a legacy function in a Vim9 script.
  */