Remove private _PyCodec_Lookup() function (#106269)

Remove the following private functions of the C API:

* _PyCodecInfo_GetIncrementalDecoder()
* _PyCodecInfo_GetIncrementalEncoder()
* _PyCodec_DecodeText()
* _PyCodec_EncodeText()
* _PyCodec_Forget()
* _PyCodec_Lookup()
* _PyCodec_LookupTextEncoding()

Move these functions to a new pycore_codecs.h internal header file.

These functions are no longer exported.