Exposes bézier handle data through a new `GreasePencilStrokePointHandle`
class on each point, which provides `position`, `type`, and `select` for both
`point.handle_left` and `point.handle_right`.
Also removes the old separate point attributes:
* `handle_left_type`, `handle_right_type`
* `select_handle_left`, `select_handle_right`
Pull Request: https://projects.blender.org/blender/blender/pulls/137780
d5b697b3f1