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.

crates/uv-cli/src/lib.rs

@@ -3763,7 +3763,7 @@ pub struct TreeArgs {
 pub struct ExportArgs {
     /// The format to which `uv.lock` should be exported.
     ///
-    /// At present, only `requirements-txt` is supported.
+    /// Supports both `requirements.txt` and `pylock.toml` (PEP 751) output formats.
     #[arg(long, value_enum, default_value_t = ExportFormat::default())]
     pub format: ExportFormat,