[flake8-pyi]: add rules for unrecognized platform check (PYI007, PYI008) (#2805)

Add two [flake8-pyi](https://github.com/PyCQA/flake8-pyi) rules (Y007, Y008). ref: #848

The specifications are described in [PEP 484 - Version and platform checking](https://peps.python.org/pep-0484/#version-and-platform-checking)

The original implementation in flake8-pyi is shown below.

- Implemention: 66f28a4407/pyi.py (L1429-L1443)
- Tests: 66f28a4407/tests/sysplatform.pyi

docs/rules/unrecognized-platform-check.md

@@ -0,0 +1,33 @@
+# unrecognized-platform-check (PYI007)
+
+Derived from the **flake8-pyi** linter.
+
+## What it does
+Check for unrecognized `sys.platform` checks. Platform checks should be
+simple string comparisons.
+
+**Note**: this rule is only enabled in `.pyi` stub files.
+
+## Why is this bad?
+Some `sys.platform` checks are too complex for type checkers to
+understand, and thus result in false positives. `sys.platform` checks
+should be simple string comparisons, like `sys.platform == "linux"`.
+
+## Example
+```python
+if sys.platform.startswith("linux"):
+   # Linux specific definitions
+else:
+  # Posix specific definitions
+```
+
+Instead, use a simple string comparison, such as `==` or `!=`:
+```python
+if sys.platform == "linux":
+    # Linux specific definitions
+else:
+    # Posix specific definitions
+```
+
+## References
+- [PEP 484](https://peps.python.org/pep-0484/#version-and-platform-checking)

docs/rules/unrecognized-platform-name.md

@@ -0,0 +1,30 @@
+# unrecognized-platform-name (PYI008)
+
+Derived from the **flake8-pyi** linter.
+
+## What it does
+Check for unrecognized platform names in `sys.platform` checks.
+
+**Note**: this rule is only enabled in `.pyi` stub files.
+
+## Why is this bad?
+If a `sys.platform` check compares to a platform name outside of a
+small set of known platforms (e.g. "linux", "win32", etc.), it's likely
+a typo or a platform name that is not recognized by type checkers.
+
+The list of known platforms is: "linux", "win32", "cygwin", "darwin".
+
+## Example
+```python
+if sys.platform == "linus":
+    ...
+```
+
+Use instead:
+```python
+if sys.platform == "linux":
+   ...
+```
+
+## References
+- [PEP 484](https://peps.python.org/pep-0484/#version-and-platform-checking)