patch 8.2.3901: Vim9: Cannot set 'cpo' in main .vimrc if using Vim9 script

Problem:    Vim9: Cannot set 'cpo' in main .vimrc if using Vim9 script.
Solution:   Do not restore 'cpo' at the end of the main .vimrc.
diff --git a/src/structs.h b/src/structs.h
index 4160453..4e770f1 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -1866,6 +1866,7 @@
     int		sn_version;	// :scriptversion
     int		sn_state;	// SN_STATE_ values
     char_u	*sn_save_cpo;	// 'cpo' value when :vim9script found
+    char	sn_is_vimrc;	// .vimrc file, do not restore 'cpo'
 
 # ifdef FEAT_PROFILE
     int		sn_prof_on;	// TRUE when script is/was profiled