gh-132983: Convert zstd __new__ methods to Argument Clinic

[AA-Turner]

• merge the tp_init functions into tp_new and convert the latter to AC. This also means we can remove the initialised member from the object structs.
• Change the signature of ZstdDict to make the content positional-only and is_raw kw-only.
• Use Zstandard where appropriate to refer to the compression algorithm, instead of the reference implementation.

A

cc @Rogdham

• Issue: Implement PEP 784 - Adding Zstandard to the Python standard library #132983