[ty] Enforce `typing.Final` (#19178)
## Summary
Emit a diagnostic when a `Final`-qualified symbol is modified. This
first iteration only works for name targets. Tests with TODO comments
were added for attribute assignments as well.
related ticket: https://github.com/astral-sh/ty/issues/158
## Ecosystem impact
Correctly identified [modification of a `Final`
symbol](https://github.com/sphinx-doc/sphinx/blob/7b4164a5f2b64781475e64daf2d05cce2a0261d8/sphinx/__init__.py#L44)
(behind a `# type: ignore`):
```diff
- warning[unused-ignore-comment] sphinx/__init__.py:44:56: Unused blanket `type: ignore` directive
```
And the same
[here](https://github.com/python-trio/trio/blob/5471a37e82b36f556e0d26b36cb95a6b05afbef1/src/trio/_core/_run.py#L128):
```diff
- warning[unused-ignore-comment] src/trio/_core/_run.py:128:45: Unused blanket `type: ignore` directive
```
## Test Plan
New Markdown tests
