api: implement object namespaces

Namespaces is a lightweight concept that should be used to group
objects for purposes of bulk operations and introspection. This is
initially used for highlights and virtual text in buffers, and is
planned to also be used for extended marks. There is no plan use them
for privileges or isolation, neither to introduce nanespace-level
options.

src/nvim/api/vim.h

@@ -4,6 +4,10 @@
 #include <stdint.h>
 
 #include "nvim/api/private/defs.h"
+#include "nvim/map.h"
+
+EXTERN Map(String, handle_T) *namespace_ids INIT(= NULL);
+EXTERN handle_T next_namespace_id INIT(= 1);
 
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "api/vim.h.generated.h"