[red-knot] Do not panic when encountering string annotations

crates/red_knot_python_semantic/resources/mdtest/annotations/string_annotations.md

@@ -0,0 +1,9 @@
+# String annotations
+
+```py
+def f() -> "int":
+    return 1
+
+# TODO: We do not support string annotations, but we should not panic if we encounter them
+reveal_type(f())  # revealed: @Todo
+```

crates/red_knot_python_semantic/src/types.rs

@@ -192,7 +192,12 @@ fn definition_expression_ty<'db>(
     if let Some(ty) = inference.try_expression_ty(expr_id) {
         ty
     } else {
-        infer_deferred_types(db, definition).expression_ty(expr_id)
+        infer_deferred_types(db, definition)
+            .try_expression_ty(expr_id)
+            .unwrap_or(
+                // Some deferred cases like string annotations are not yet implemented
+                Type::Todo,
+            )
     }
 }