API: Refactor: Register/unregister created/destroyed buffers

- Add the 'handle' field to `buf_T`
- Add declare/implement functions for registering/unregistering/retrieving
  buffers
- Register/unregister buffers when they are created/destroyed.

src/nvim/buffer_defs.h

@@ -401,6 +401,7 @@ typedef struct {
  */
 
 struct file_buffer {
+  uint64_t handle;              // unique identifier for the buffer
   memline_T b_ml;               /* associated memline (also contains line
                                    count) */