Migrate to monorepo with higlass-schema subpackage

[manzt]

This PR migrates higlass-schema from its own repository into the higlass-python monorepo using uv workspaces. Previously, higlass-python depended on higlass-schema as an external PyPI package. Now both packages live together in a monorepo, allowing us to develop and test them in tandem.

❯ tree --level=3
.
├── docs/
├── examples/
├── packages
│   └── higlass-schema
│       ├── src
│       ├── tests
│       ├── LICENSE
│       ├── pyproject.toml
│       ├── README.md
│       └── uv.lock
├── src/
├── test/
├── deno.json
├── LICENSE
├── PR.md
├── pyproject.toml
├── README.md
└── uv.lock

The main benefit is that we can make changes to both packages atomically - no more coordinating releases just to test integration changes. Both packages share the same linting, formatting, testing setup.

TODO:

• Figure out the release strategy for publishing multiple packages from the same repository. We'll likely use prefixed tags (e.g., higlass-python-v1.0.0, higlass-schema-v1.0.0) with automated PyPI publishing.

Once this is merged, we can archive the original higlass-schema repository.