n-api: clean up binding creation

* Remove dead code for `GetterCallbackWrapper` and
  `SetterCallbackWrapper`.
* Factor out creation of new `v8::Function`s.
* Factor out creation of new `v8::FunctionTemplate`s.
* Turn `CallbackBundle` into a class, internalizing creation of new
  instances and garbage collection.

Signed-off-by: Gabriel Schulhof <gabriel.schulhof@intel.com>
PR-URL: https://github.com/nodejs/node/pull/36170


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Michael Dawson <midawson@redhat.com>