Fix conversion to array

[awni]

Closes #1066

This is kind of a nanobind issue but one that they have refused to fix in the past.

• Basically a np.ndarray is implicitly convertible to a std::complex<float> so our to_array was allowing that.
• Rather than disallowing ops with np.ndarrays instead I added it as a member in the variant.
• That however has side effects because now mx.array gets implicitly converted to nb::ndarray because of the buffer protocol.
• So to fix that I also had to enable array as a member of the variant.