Add flake8-pyi with one rule (#2682)

Add basic scaffold for [flake8-pyi](https://github.com/PyCQA/flake8-pyi) and the first rule, Y001

rel: https://github.com/charliermarsh/ruff/issues/848

docs/rules/prefix-type-params.md

@@ -0,0 +1,25 @@
+# prefix-type-params (PYI001)
+
+Derived from the **flake8-pyi** linter.
+
+### What it does
+Checks that type `TypeVar`, `ParamSpec`, and `TypeVarTuple` definitions in
+stubs are prefixed with `_`.
+
+### Why is this bad?
+By prefixing type parameters with `_`, we can avoid accidentally exposing
+names internal to the stub.
+
+### Example
+```python
+from typing import TypeVar
+
+T = TypeVar("T")
+```
+
+Use instead:
+```python
+from typing import TypeVar
+
+_T = TypeVar("_T")
+```