Support PEP 739 (build-details.json)

I came across PEP 739 (https://peps.python.org/pep-0739/), which says that, starting in Python 3.14, there will be a build-details.json that gives information about the Python runtime.

There's a variety of things in it. Most of them are relevant to defining a toolchain.

For local toolchains, it looks like it can be used to get the necessary information for the toolchain _without_ running Python.

For remote toolchains, it seems less useful, but there's some info that might still be helpful. It's less useful because things like e.g. Python version have to be known before the repo that triggers a download is referenced. Populating the abi flags some of that lower level info would still be helpful, though.

* language.version_info -- Tells which Python versio
* abi.flags, extension_suffix, implementation
* libpython
* c_api