[ty] Support as-patterns in reachability analysis (#19728)

## Summary

Support `as` patterns in reachability analysis:

```py
from typing import assert_never


def f(subject: str | int):
    match subject:
        case int() as x:
            pass
        case str():
            pass
        case _:
            assert_never(subject)  # would previously emit an error
```

Note that we still don't support inferring correct types for the bound
name (`x`).

Closes https://github.com/astral-sh/ty/issues/928

## Test Plan

New Markdown tests

crates/ty_python_semantic/src/semantic_index/builder.rs

@@ -838,6 +838,13 @@ impl<'db, 'ast> SemanticIndexBuilder<'db, 'ast> {
                     .collect();
                 PatternPredicateKind::Or(predicates)
             }
+            ast::Pattern::MatchAs(pattern) => PatternPredicateKind::As(
+                pattern
+                    .pattern
+                    .as_ref()
+                    .map(|p| Box::new(self.predicate_kind(p))),
+                pattern.name.as_ref().map(|name| name.id.clone()),
+            ),
             _ => PatternPredicateKind::Unsupported,
         }
     }