api/msgpack-rpc: Refactor metadata object construction

Instead of building all metadata from msgpack-gen.lua, we now merge the
generated part with manual information(such as types and features). The metadata
is accessible through the api method `vim_get_api_info`.

This was done to simplify the generator while also increasing flexibility(by
being able to add more metadata)

src/nvim/api/private/defs.h

@@ -44,13 +44,9 @@ typedef struct {
 } Dictionary;
 
 typedef enum {
-// The following comments are markers that msgpack-gen.lua uses to extract
-// types, don't remove!
-// start custom types
   kObjectTypeBuffer,
   kObjectTypeWindow,
   kObjectTypeTabpage,
-// end custom types
   kObjectTypeNil,
   kObjectTypeBoolean,
   kObjectTypeInteger,