GH-131238: Core header refactor

[markshannon]

This PR moves most of the struct definitions in the various "core" header files to new header files.
This breaks the dependencies between header files, as headers do not need to include each other, but can include one or two of the struct definition headers.

• Issue: Our internal headers are far too interlinked #131238

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @markshannon for commit 4231362 🤖

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https://buildbot.python.org/all/#/grid?branch=refs%2Fpull%2F131250%2Fmerge

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[vstinner]

Please add also the 3 new internal headers to PCbuild/pythoncore.vcxproj and PCbuild/pythoncore.vcxproj.filters.

[vstinner]

LGTM. I like the overall design. I just left a few minor comments which can be addressed later if needed (I can handle them if you want).