bpo-45316: Move private PyCode C API to internal C API (GH-31576)

Rename private functions (no exported), add an underscore prefix:

* PyLineTable_InitAddressRange() => _PyLineTable_InitAddressRange()
* PyLineTable_NextAddressRange() => _PyLineTable_NextAddressRange()
* PyLineTable_PreviousAddressRange() => _PyLineTable_PreviousAddressRange()

Move private functions to the internal C API:

* _PyCode_Addr2EndLine()
* _PyCode_Addr2EndOffset()
* _PyCode_Addr2Offset()
* _PyCode_InitAddressRange()
* _PyCode_InitEndAddressRange(
* _PyLineTable_InitAddressRange()
* _PyLineTable_NextAddressRange()
* _PyLineTable_PreviousAddressRange()

No longer export the following internal functions:

* _PyCode_GetVarnames()
* _PyCode_GetCellvars()
* _PyCode_GetFreevars()
* _Py_GetSpecializationStats()

Add "extern" to pycore_code.h functions to identify them more easiliy
(they are still not exported).