feat: Add ty type checker support

[alec-bike]

This concept PR adds initial ty type checker support to altair. To keep ty separate from existing mypy (and pyright), a new ty.toml configuration file is used for now.

The PR addresses the following issues:

• peg ruff and mypy to latest version, fix associated errors
• remove mypy-specific annotations from type: ignore (see note below)
• fix minor type issues
• remove some Union and pyright: ignore that are no longer needed
• add some ty: ignore to flag issues that do not yet have a fix

Limitations:

• only check altair/vegalite
• exclude altair/vegalite/v6/schema
• no ty support in CI

mypy-specific annotations

Annotations such as type: ignore[arg-type] are no longer treated as type: ignore by ty. Converting these to generic type: ignore does work with ty (and should with most other type checkers).

There are discussions around cross-compatibility between mypy and ty (see astral-sh/ty#3127), which would obviate this issue in some cases.

Alternatively, ty-specific annotations could be added (type: ignore[arg-type, ty:invalid-argument-type]), but this is quite verbose (and requires adding more annotations if future type checkers are adopted).

This PR proposes reverting to the generic type: ignore by removing some existing mypy-specific annotations. @mattijn and @dangotbanned, we could use this concept PR as a sounding board to discuss whether this is the best approach.