[red-knot] Support for TOML configs in Markdown tests (#14785)
## Summary
This adds support for specifying the target Python version from a
Markdown test. It is a somewhat limited ad-hoc solution, but designed to
be future-compatible. TOML blocks can be added to arbitrary sections in
the Markdown block. They have the following format:
````markdown
```toml
[tool.knot.environment]
target-version = "3.13"
```
````
So far, there is nothing else that can be configured, but it should be
straightforward to extend this to things like a custom typeshed path.
This is in preparation for the statically-known branches feature where
we are going to have to specify the target version for lots of tests.
## Test Plan
- New Markdown test that fails without the explicitly specified
`target-version`.
- Manually tested various error paths when specifying a wrong
`target-version` field.
- Made sure that running tests is as fast as before.
