[red-knot] Implicit instance attributes (#15811)
## Summary
Add support for implicitly-defined instance attributes, i.e. support
type inference for cases like this:
```py
class C:
def __init__(self) -> None:
self.x: int = 1
self.y = None
reveal_type(C().x) # int
reveal_type(C().y) # Unknown | None
```
## Benchmarks
Codspeed reports no change in a cold-cache benchmark, and a -1%
regression in the incremental benchmark. On `black`'s `src` folder, I
don't see a statistically significant difference between the branches:
| Command | Mean [ms] | Min [ms] | Max [ms] | Relative |
|:---|---:|---:|---:|---:|
| `./red_knot_main check --project /home/shark/black/src` | 133.7 ± 9.5 | 126.7 | 164.7 | 1.01 ± 0.08 |
| `./red_knot_feature check --project /home/shark/black/src` | 132.2 ± 5.1 | 118.1 | 140.9 | 1.00 |
## Test Plan
Updated and new Markdown tests
semantic_index.rs