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.)

Modules/_testinternalcapi.c

@@ -550,6 +550,51 @@ _testinternalcapi_optimize_cfg_impl(PyObject *module, PyObject *instructions,
 }
 
 
+static PyObject *
+get_interp_settings(PyObject *self, PyObject *args)
+{
+    int interpid = -1;
+    if (!PyArg_ParseTuple(args, "|i:get_interp_settings", &interpid)) {
+        return NULL;
+    }
+
+    PyInterpreterState *interp = NULL;
+    if (interpid < 0) {
+        PyThreadState *tstate = _PyThreadState_GET();
+        interp = tstate ? tstate->interp : _PyInterpreterState_Main();
+    }
+    else if (interpid == 0) {
+        interp = _PyInterpreterState_Main();
+    }
+    else {
+        PyErr_Format(PyExc_NotImplementedError,
+                     "%zd", interpid);
+        return NULL;
+    }
+    assert(interp != NULL);
+
+    PyObject *settings = PyDict_New();
+    if (settings == NULL) {
+        return NULL;
+    }
+
+    /* Add the feature flags. */
+    PyObject *flags = PyLong_FromUnsignedLong(interp->feature_flags);
+    if (flags == NULL) {
+        Py_DECREF(settings);
+        return NULL;
+    }
+    int res = PyDict_SetItemString(settings, "feature_flags", flags);
+    Py_DECREF(flags);
+    if (res != 0) {
+        Py_DECREF(settings);
+        return NULL;
+    }
+
+    return settings;
+}
+
+
 static PyMethodDef TestMethods[] = {
     {"get_configs", get_configs, METH_NOARGS},
     {"get_recursion_depth", get_recursion_depth, METH_NOARGS},
@@ -569,6 +614,7 @@ static PyMethodDef TestMethods[] = {
     {"set_eval_frame_default", set_eval_frame_default, METH_NOARGS, NULL},
     {"set_eval_frame_record", set_eval_frame_record, METH_O, NULL},
     _TESTINTERNALCAPI_OPTIMIZE_CFG_METHODDEF
+    {"get_interp_settings", get_interp_settings, METH_VARARGS, NULL},
     {NULL, NULL} /* sentinel */
 };