Support type alias statements in simple statement positions (#8916)

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## Summary

Our `SoftKeywordTokenizer` only respected soft keywords in compound
statement positions -- for example, at the start of a logical line:

```python
type X = int
```

However, type aliases can also appear in simple statement positions,
like:

```python
class Class: type X = int
```

(Note that `match` and `case` are _not_ valid keywords in such
positions.)

This PR upgrades the tokenizer to track both kinds of valid positions.

Closes https://github.com/astral-sh/ruff/issues/8900.
Closes https://github.com/astral-sh/ruff/issues/8899.

## Test Plan

`cargo test`

crates/ruff_python_parser/src/parser.rs

@@ -822,6 +822,10 @@ type X \
     [T] = T
 type X[T] \
     = T
+
+# simple statements
+type X = int; type X = str; type X = type
+class X: type X = int
 "#;
         insta::assert_debug_snapshot!(parse_suite(source, "<test>").unwrap());
     }
@@ -859,10 +863,17 @@ type (
 type = 1
 type = x = 1
 x = type = 1
+lambda x: type
 ";
         insta::assert_debug_snapshot!(parse_suite(source, "<test>").unwrap());
     }
 
+    #[test]
+    fn test_invalid_type() {
+        assert!(parse_suite("a: type X = int", "<test>").is_err());
+        assert!(parse_suite("lambda: type X = int", "<test>").is_err());
+    }
+
     #[test]
     fn numeric_literals() {
         let source = r"x = 123456789