[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](7b4164a5f2/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](5471a37e82/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

crates/ty_python_semantic/src/semantic_index/definition.rs

@@ -618,6 +618,15 @@ impl DefinitionKind<'_> {
         }
     }
 
+    pub(crate) fn is_import(&self) -> bool {
+        matches!(
+            self,
+            DefinitionKind::Import(_)
+                | DefinitionKind::ImportFrom(_)
+                | DefinitionKind::StarImport(_)
+        )
+    }
+
     /// Returns the [`TextRange`] of the definition target.
     ///
     /// A definition target would mainly be the node representing the place being defined i.e.,