patch 8.2.4617: no completion for :scriptnames

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

src/usercmd.c

@@ -93,6 +93,7 @@ static struct
     {EXPAND_USER_VARS, "var"},
 #if defined(FEAT_EVAL)
     {EXPAND_BREAKPOINT, "breakpoint"},
+    {EXPAND_SCRIPTNAMES, "scriptnames"},
 #endif
     {0, NULL}
 };