gh-106078: Convert `_decimal` types to heap types (#106079)

- Establish global state struct
- Convert static types to heap types and add them to global state:
    * PyDecContextManager_Type
    * PyDecContext_Type
    * PyDecSignalDictMixin_Type
    * PyDec_Type
- Add to global state:
    * PyDecSignalDict_Type
    * DecimalTuple

Co-authored-by: Kumar Aditya <59607654+kumaraditya303@users.noreply.github.com>
Co-authored-by: Erlend E. Aasland <erlend@python.org>