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.c

@@ -100,7 +100,8 @@ void estack_pop(void)
 }
 
 /// Get the current value for <sfile> in allocated memory.
-/// @param which  ESTACK_SFILE for <sfile> and ESTACK_STACK for <stack>.
+/// @param which  ESTACK_SFILE for <sfile>, ESTACK_STACK for <stack> or
+///               ESTACK_SCRIPT for <script>.
 char *estack_sfile(estack_arg_T which)
 {
   estack_T *entry = ((estack_T *)exestack.ga_data) + exestack.ga_len - 1;
@@ -111,6 +112,27 @@ char *estack_sfile(estack_arg_T which)
     return xstrdup(entry->es_name);
   }
 
+  // If evaluated in a function return the path of the script where the
+  // function is defined, at script level the current script path is returned
+  // instead.
+  if (which == ESTACK_SCRIPT) {
+    if (entry->es_type == ETYPE_UFUNC) {
+      sctx_T *def_ctx = &entry->es_info.ufunc->uf_script_ctx;
+      if (def_ctx->sc_sid > 0) {
+        return xstrdup((char *)(SCRIPT_ITEM(def_ctx->sc_sid).sn_name));
+      }
+    } else if (exestack.ga_len > 0) {
+      // Walk the stack backwards, starting from the current frame.
+      for (int idx = exestack.ga_len - 1; idx; idx--) {
+        entry = ((estack_T *)exestack.ga_data) + idx;
+        if (entry->es_type == ETYPE_SCRIPT) {
+          return xstrdup(entry->es_name);
+        }
+      }
+    }
+    return NULL;
+  }
+
   // Give information about each stack entry up to the root.
   // For a function we compose the call stack, as it was done in the past:
   //   "function One[123]..Two[456]..Three"