[`flake8-pyi`] Implement PYI026 (#5844)
## Summary
Checks for `typehint.TypeAlias` annotation in type aliases. See
[original
source](https://github.com/PyCQA/flake8-pyi/blob/main/pyi.py#L1085).
```
$ flake8 --select Y026 crates/ruff/resources/test/fixtures/flake8_pyi/PYI026.pyi
crates/ruff/resources/test/fixtures/flake8_pyi/PYI026.pyi:4:1: Y026 Use typing_extensions.TypeAlias for type aliases, e.g. "NewAny: TypeAlias = Any"
crates/ruff/resources/test/fixtures/flake8_pyi/PYI026.pyi:5:1: Y026 Use typing_extensions.TypeAlias for type aliases, e.g. "OptinalStr: TypeAlias = typing.Optional[str]"
crates/ruff/resources/test/fixtures/flake8_pyi/PYI026.pyi:6:1: Y026 Use typing_extensions.TypeAlias for type aliases, e.g. "Foo: TypeAlias = Literal['foo']"
crates/ruff/resources/test/fixtures/flake8_pyi/PYI026.pyi:7:1: Y026 Use typing_extensions.TypeAlias for type aliases, e.g. "IntOrStr: TypeAlias = int | str"
crates/ruff/resources/test/fixtures/flake8_pyi/PYI026.pyi:8:1: Y026 Use typing_extensions.TypeAlias for type aliases, e.g. "AliasNone: TypeAlias = None"
```
```
$ ./target/debug/ruff --select PYI026 crates/ruff/resources/test/fixtures/flake8_pyi/PYI026.pyi --no-cache
crates/ruff/resources/test/fixtures/flake8_pyi/PYI026.pyi:4:1: PYI026 Use `typing.TypeAlias` for type aliases in `NewAny`, e.g. "NewAny: typing.TypeAlias = Any"
crates/ruff/resources/test/fixtures/flake8_pyi/PYI026.pyi:5:1: PYI026 Use `typing.TypeAlias` for type aliases in `OptinalStr`, e.g. "OptinalStr: typing.TypeAlias = typing.Optional[str]"
crates/ruff/resources/test/fixtures/flake8_pyi/PYI026.pyi:6:1: PYI026 Use `typing.TypeAlias` for type aliases in `Foo`, e.g. "Foo: typing.TypeAlias = Literal["foo"]"
crates/ruff/resources/test/fixtures/flake8_pyi/PYI026.pyi:7:1: PYI026 Use `typing.TypeAlias` for type aliases in `IntOrStr`, e.g. "IntOrStr: typing.TypeAlias = int | str"
crates/ruff/resources/test/fixtures/flake8_pyi/PYI026.pyi:8:1: PYI026 Use `typing.TypeAlias` for type aliases in `AliasNone`, e.g. "AliasNone: typing.TypeAlias = None"
Found 5 errors.
```
ref: #848
## Test Plan
Snapshots, manual runs of flake8.
