(Note that this PR adds "private" symbols. We'll probably make them public, and add docs, in a separate change.)

We do the following:

1. change the argument to a new _PyInterpreterConfig struct
2. rename the function to _Py_NewInterpreterFromConfig(), inspired by Py_InitializeFromConfig() (takes a _PyInterpreterConfig instead of isolated_subinterpreter)
3. split up the boolean isolated_subinterpreter into the corresponding multiple granular settings
   • allow_fork
   • allow_subprocess
   • allow_threads
4. add PyInterpreterState.feature_flags to store those settings
5. add a function for checking if a feature is enabled on an opaque PyInterpreterState *
6. drop PyConfig._isolated_interpreter

The current default (Py_NewInterpeter() and Py_Initialize*()) allows fork, subprocess, and threads and the optional "isolated" interpreter disables all three. None of that changes here.

Note that the given _PyInterpreterConfig will not be used (nor preserved, for now) outside _Py_NewInterpreterFromConfig(). This contrasts with how PyConfig is currently preserved, used, and modified outside Py_InitializeFromConfig(). I'd rather just avoid that mess from the start for _PyInterpreterConfig.

This change allows us to follow up with a number of improvements (e.g. stop disallowing subprocess and support disallowing exec instead).

• Issue: Expose _Py_NewInterpreter() as Py_NewInterpreterFromConfig() #98608