gh-98608: Change _Py_NewInterpreter() to _Py_NewInterpreterFromConfig() (gh-98609)

(see https://github.com/python/cpython/issues/98608)

This change does 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 existing default (see `Py_NewInterpeter()` and `Py_Initialize*()`) allows fork, subprocess, and threads and the optional "isolated" interpreter (see the `_xxsubinterpreters` module) disables all three.  None of that changes here; the defaults are preserved.

Note that the given `_PyInterpreterConfig` will not be used outside `_Py_NewInterpreterFromConfig()`, nor preserved.  This contrasts with how `PyConfig` is currently preserved, used, and even modified outside `Py_InitializeFromConfig()`.  I'd rather just avoid that mess from the start for `_PyInterpreterConfig`.  We can preserve it later if we find an actual need.

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

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

Include/internal/pycore_interp.h

@@ -143,6 +143,7 @@ struct _is {
 #ifdef HAVE_DLOPEN
     int dlopenflags;
 #endif
+    unsigned long feature_flags;
 
     PyObject *dict;  /* Stores per-interpreter state */