completion: Add v:completed_item feature #2563

Reviewed-by: Michael Reed <m.reed@mykolab.com>
Reviewed-by: Luke Andrew <luke.github@la.id.au>
Reviewed-by: Justin M. Keyes <justinkz@gmail.com>
Reviewed-by: Florian Walch <florian@fwalch.com>

src/nvim/eval.h

@@ -64,6 +64,7 @@ enum {
     VV_WINDOWID,
     VV_PROGPATH,
     VV_COMMAND_OUTPUT,
+    VV_COMPLETED_ITEM,
     VV_LEN, /* number of v: vars */
 };