BLD/CI: switch to uv_build, add windows and macOS CI - #21

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683, prefix-dev/setup-pixi@92815284c57faa15cd896c4d5cfb2d59f32dc43d. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/