Normalize marker expression order (astral-sh#5422)

## Summary

Normalize the order of marker expressions on construction. This removes
the distinction between expressions like `os_name == 'Linux'` vs.
`'Linux' == os_name` throughout the codebase. One caveat here is that
the `in` operator does not have a direct inverse, so we introduce
`MarkerOperator::Contains` to handle that case.

I wanted to land this smaller change before some more intrusive changes
as it simplifies the existing code quite a bit.

Bump version to 0.2.28 (astral-sh#5363)

Bump version to 0.2.27 (astral-sh#5230)

Bump version to v0.2.26 (astral-sh#5149)

Bump version to v0.2.25 (astral-sh#5083)

Bump version to 0.2.24 (astral-sh#4974)

Bump version to 0.2.23 (astral-sh#4903)

Bump version to v0.2.22 (astral-sh#4862)

Bump version to 0.2.21 (astral-sh#4757)

Bump version to 0.2.20 (astral-sh#4745)