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/usercmd.c

@@ -96,6 +96,7 @@ static const char *command_complete[] = {
   [EXPAND_USER] = "user",
   [EXPAND_USER_VARS] = "var",
   [EXPAND_BREAKPOINT] = "breakpoint",
+  [EXPAND_SCRIPTNAMES] = "scriptnames",
 };
 
 /// List of names of address types.  Must be alphabetical for completion.