Support for PEP 794 (import name metadata)

[PEP 794](https://peps.python.org/pep-0794/) adds `import-names` and `import-namespaces` keys under `[project]` in `pyproject.toml`. Support for that just landed in `pyproject-metadata`, see https://github.com/pypa/pyproject-metadata/pull/260. Once that is released, we can add support.

EDIT: prerequisites are support in:

- [x] `pyproject-metadata` (0.10.0, see https://github.com/pypa/pyproject-metadata/pull/260)
- [x] `packaging` (26.0, see https://github.com/pypa/packaging/issues/950)
- [x] PyPI (see https://github.com/pypi/warehouse/issues/19083)
- [x] `twine` (no issue yet)
- [x] `uv build` (0.11.17, see https://github.com/astral-sh/uv/issues/16435)

Start by reading PEP 794 and reviewing how meson-python consumes pyproject-metadata for the [project] table. Add support for the import-names and import-namespaces keys once the listed prerequisites are available, then verify that these values are handled during the build-backend flow.

Written by the indexing model from the issue text.