Re-enable Python bindings for Properties::Type (fixes the Blender plugin)

[dvicini]

This PR re-enables the Python bindings for Properties::Type. The motivation for this is that the Blender importer queries the types of properties (e.g., https://github.com/mitsuba-renderer/mitsuba-blender/blob/5a12adda2d0da3ccf3c125a54e36f7cf57988957/mitsuba-blender/io/importer/emitters.py#L52), which currently is broken.

One minor complication is that the enum is nested in Properties, which itself is variant dependent. However, the enum isn't. I therefore added some logic to detect if the type was already bound, similar to the MI_PY_CHECK_ALIAS macro. I am not sure if this is the best option, but it seems to work.

I also modified the unit tests to exercise the functionality

[dvicini]

One comment: I am not sure if the binding still leaks as referenced in the original comment. Maybe @njroussel would know how to check this?

[wjakob]

I suspect the leaks are a remnant from an older version of nanobind, where enumerations were done with nb::class_<>, which often caused leaks due to circular references. The change looks good, I will merge it. Unrelated: there is some refactoring to Properties planned (refactor branch) that will likely need some careful attention to the Blender plugin to avoid breaking things.