patch 8.2.4617: no completion for :scriptnames

Problem:    No completion for :scriptnames.
Solution:   Implement :scriptnames completion. (Yegappan Lakshmanan,
            closes #10005)
diff --git a/src/vim.h b/src/vim.h
index d7a2464..0a923f1 100644
--- a/src/vim.h
+++ b/src/vim.h
@@ -802,6 +802,7 @@
 #define EXPAND_DIFF_BUFFERS	49
 #define EXPAND_DISASSEMBLE	50
 #define EXPAND_BREAKPOINT	51
+#define EXPAND_SCRIPTNAMES	52
 
 // Values for exmode_active (0 is no exmode)
 #define EXMODE_NORMAL		1