[red-knot] Move intersection type tests to Markdown (#15396)
## Summary
[**Rendered version of the new test
suite**](https://github.com/astral-sh/ruff/blob/david/intersection-type-tests/crates/red_knot_python_semantic/resources/mdtest/intersection_types.md)
Moves most of our existing intersection-types tests to a dedicated
Markdown test suite, extends the test coverage, unifies the notation for
these tests, groups tests into a proper structure, and adds some
explanations for various simplification strategies.
This changeset also:
- Adds a new simplification where `~Never` is removed from
intersections.
- Adds a new simplification where adding `~object` simplifies the whole
intersection to `Never`
- Avoids unnecessary assignment-checks between inferred and declared
type. This was added to this changeset to avoid many false positive
errors in this test suite.
Resolves the task described in this old comment
[here](https://github.com/astral-sh/ruff/pull/13962/files/e01da82a5a0ef6a2af0aa4dc50f898cffadb4a33..e7e432bca2b3f24979da55a9a34ad765aaaae8d1#r1819924085).
## Test Plan
Running the new Markdown tests
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Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
