vim-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 vim/vim#10121)
6013d0045d

Use `.sn_name` instead of `->sn_name` as v8.2.0154 hasn't been ported.
Cherry-pick builtin.txt expand() doc from latest Vim.

src/nvim/runtime.h

@@ -47,6 +47,7 @@ typedef enum {
   ESTACK_NONE,
   ESTACK_SFILE,
   ESTACK_STACK,
+  ESTACK_SCRIPT,
 } estack_arg_T;
 
 typedef struct scriptitem_S {