Prototype nanobind bindings for Imath

We have the long term goal of removing the dependency on Boost.Python, and the current strategy is to rewrite the bindings with pybind11, although [nanobind](https://github.com/wjakob/nanobind) promises to be even better than pybind11, and also claims to be largely API-compatible.

We should prototype bindings using nanobind. This would involve duplicating the current state of the pybind11 bindings, both source and CMake configuration, and replacing both to use nanobind.

Start by locating the current pybind11 bindings and their CMake configuration, then compare their source and build setup with the proposed nanobind approach. Done means a prototype that duplicates the current binding state while replacing pybind11 usage with nanobind in both source and CMake configuration.

Written by the indexing model from the issue text.