Add Py_PACK_VERSION macros

[encukou]

Add these to limited API:

• Py_PACK_FULL_VERSION(x, y, z, level, serial): pack a version number from components
  into the format used by Py_VERSION_HEX and Py_LIMITED_API.
  For example, Py_PACK_FULL_VERSION(3, 14, 0, 0xA, 1) evaluates to 0x030E00A1.

• Py_PACK_VERSION(x, y): shorthand for Py_PACK_FULL_VERSION(x, y, 0, 0, 0),
  useful because the first two version components often determine ABI
  compatibility.

These are primarily macros, but we will export library functions with the same names and functionality, for use in wrappers for non-C languages – for example, Python with ctypes.

The functions take int arguments and return uint32_t. Inputs are masked (high bits are ignored).

C API WG decision: capi-workgroup/decisions#47

Linked PRs

• gh-128629: Add Py_PACK_VERSION and Py_PACK_FULL_VERSION #128630