Add `uv export` support for PEP 751 (#12955)
## Summary
This PR adds `uv export` support for [PEP
751](https://peps.python.org/pep-0751). We don't yet expose a way to
consume the generated lockfile, but it's a first step.
The logic to go from `uv.lock` to "flat set of packages to include, with
markers telling us when to include them" is all shared with the
`requirements.txt` export (and extracted in
https://github.com/astral-sh/uv/pull/12956). So most of the code is just
converting from our internal types to the PEP 751 schema.
