Add TypeChecker.forAny(checkers)

This allows teams to deprecate one API and prefer the other. For example:

```dart
const $Foo = const TypeChecker.fromRuntime(Foo);
const $Bar = const TypeChecker.fromRuntime(Bar);

// Used until $Foo is deleted.
const $FooOrBar = . const TypeChecker.forAny(const [$Foo, $Bar]);
```