patch 8.2.4726: cannot use expand() to get the script name

Problem:    Cannot use expand() to get the script name.
Solution:   Support expand('<script>'). (closes #10121)
diff --git a/src/vim.h b/src/vim.h
index f9ec6e5..0dcadd1 100644
--- a/src/vim.h
+++ b/src/vim.h
@@ -2220,7 +2220,8 @@
 typedef enum {
     ESTACK_NONE,
     ESTACK_SFILE,
-    ESTACK_STACK
+    ESTACK_STACK,
+    ESTACK_SCRIPT,
 } estack_arg_T;
 
 // Flags for assignment functions.