Add more options for type parameters

It would be great to have more options for type parameters. Specifically, it would be helpful for me if you could specify multiple types with an `or` and if you could make the type parameter `required`.

Example:
``` dart
class MyClass {}

class MyClass {}

// good
final a = MyClass();

// bad => linter says type parameter is required
final a = MyClass();
```

No implementation files or tests are named. Start by reviewing the Dart language specification and the existing rules for type parameters, then determine how the proposed `or` bounds and `required` type parameters would fit those rules. Done means the language design, syntax, semantics, and validation approach are agreed.

Written by the indexing model from the issue text.