Writing to the `curve_type` attribute directly is not allowed as there are other
updates needed and otherwise will result in a crash.
The fix makes sure the `curve_type` is read-only. To change the curve type,
the `grease_pencil.set_curve_type` operator has to be used for now.
This moves the helper python classes from `scripts/modules/grease_pencil_python.py`
to `scripts/modules/_bpy_internal/grease_pencil.py`.
It also cleans up the code a bit more. No functional changes.
Pull Request: https://projects.blender.org/blender/blender/pulls/126403