gh-106320: Remove private _PyDict functions (#108449)

Move private functions to the internal C API (pycore_dict.h):

* _PyDictView_Intersect()
* _PyDictView_New()
* _PyDict_ContainsId()
* _PyDict_DelItemId()
* _PyDict_DelItem_KnownHash()
* _PyDict_GetItemIdWithError()
* _PyDict_GetItem_KnownHash()
* _PyDict_HasSplitTable()
* _PyDict_NewPresized()
* _PyDict_Next()
* _PyDict_Pop()
* _PyDict_SetItemId()
* _PyDict_SetItem_KnownHash()
* _PyDict_SizeOf()

No longer export most of these functions.

Move also the _PyDictViewObject structure to the internal C API.

Move dict_getitem_knownhash() function from _testcapi to the
_testinternalcapi extension. Update test_capi.test_dict for this
change.