Add infer keyword

Let:
```
abstract class Entity {
T get id;
}

class EntityStore, K> {
T getById(K id) {
...
}
}

class Customer extends Entity {
final int id;
Customer(this.id);
}
```

Calling Code:
`final store = EntityStore();`

But type parameter `K` could already be inferred from `T`:
`final store = EntityStore(); // this doesn't work`

I propose `infer` keyword for type parameters that could be inferred from other type parameters:
`class EntityStore, infer K> {}`

Start with the examples in issue #2173 and read its 13-comment thread to understand the proposed infer keyword and the type relationship it targets. Done means the language design has a resolved decision and documented behavior for the EntityStore examples.

Written by the indexing model from the issue text.