Add structured version info for decimal

Feature or enhancement

decimal.__libmpdec_version__ (from mpd_version()) is the only information about the libmpdec library, and it only describes the library loaded at runtime. The version of the library that the module was built against is not exposed, although it can differ now that only a system libmpdec is used.

libmpdec provides MPD_VERSION, MPD_MAJOR_VERSION, MPD_MINOR_VERSION and MPD_MICRO_VERSION at compile time, and mpd_version() at runtime.

Add decimal.LIBMPDEC_VERSION and decimal.LIBMPDEC_VERSION_INFO for the build-time version, decimal.libmpdec_version as an alias of __libmpdec_version__, and decimal.libmpdec_version_info for the runtime version.

These names are specific to the C implementation and are not added to _pydecimal; the test which checks that both implementations expose the same names has to exclude them.

Linked PRs

• gh-157476

先找出 decimal 的 C 實作和 _pydecimal 實作，然後檢查現有的 mpd_version() 和 libmpdec_version 處理方式。檢查要求兩個實作公開相同名稱的測試；當 C 實作公開所要求的建置時期和執行階段名稱，且一致性測試排除這些名稱時，即表示完成。