Add pyproject.toml validation to pre-commit

[AlexWaygood]

Add validate-pyproject to check the contents of pyproject.toml files in addition to the syntax checks provided by check-toml, using the same hook and dependency pins as typing_extensions.

It validates standard project metadata and build-system settings, plus supported tool configuration through bundled SchemaStore schemas. For example, it catches:

• Invalid project metadata, such as version = 1 instead of a string or a malformed Python version constraint like requires-python = "=>3.10".
• Incorrect build-system settings, such as requires = "hatchling" instead of a list of requirements.
• Misspelled tool settings and incorrect value types, such as line-lenght = 130 under [tool.ruff] or preview = "true" instead of a boolean under [tool.black].

Assisted by Codex.

[srittau]

I think we're getting in the territory where the potential threat through compromised third-party actions outweighs the usefulness of the the checks. pyproject.toml barely changes, these changes are peer reviewed, and most errors should be spotted pretty quickly because changes have no effect. Overall, I'm -0 on this.

[AlexWaygood]

yeah, this one's pretty marginal, I don't feel strongly about this one!