Add compile check for interface down-cast without explicit implementation

[eladb]

Repro:

class MyClass {
  foo: number
}

interface Fooable {
  foo: number;
}

var f: Fooable = new MyClass() as Fooable;

This works in TypeScript but will fail (at runtime) in Java.

See: aws/aws-cdk#504, aws/aws-cdk#505

[eladb]

This might be very hard to validate since we only look at public APIs

[RomainMuller]

Technically we have access to everything, so it can be done. But it won't be trivial for sure.

[github-actions]

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