Detect free-threaded Python from configuration

Per the work @rgommers did in https://github.com/mesonbuild/meson/pull/13338 it appears that the Meson configuration internally detects when it is using a free-threaded interpreter and does the right thing from there

Is there any API through which the user can detect this in their own configuration? I am seeing a use case in nanobind where that build system defines a macro depending on whether or not a free-threaded interpreter is detected.

Start by reviewing the free-threaded interpreter detection described in Meson pull request 13338 and trace how that configuration state is exposed. Determine the user-facing API and its expected behavior for the nanobind use case, then add tests showing that a configuration can reliably detect the interpreter mode.

Written by the indexing model from the issue text.