vim-patch:8.2.4617: no completion for :scriptnames

Problem:    No completion for :scriptnames.
Solution:   Implement :scriptnames completion. (Yegappan Lakshmanan,
            closes vim/vim#10005)

454ce6737c

Co-authored-by: Yegappan Lakshmanan <yegappan@yahoo.com>

src/nvim/vim.h

@@ -155,6 +155,7 @@ enum {
   EXPAND_ARGLIST,
   EXPAND_DIFF_BUFFERS,
   EXPAND_BREAKPOINT,
+  EXPAND_SCRIPTNAMES,
   EXPAND_CHECKHEALTH,
   EXPAND_LUA,
 };