Support Java 17 sealed types

[marek-parfianowicz]

Sealed types was a preview feature in Java 15.
It became part of the language in Java 17.

Sealed types is a feature introduced in Java to enhance the control and security of class access and inheritance.

• Sealed types restrict which other classes or interfaces can extend or implement them.
• When you declare a class as sealed, you specify a limited set of classes or interfaces that can be direct subtypes of the sealed class.
• This ensures that only authorized classes can inherit from the sealed class.
• You define the permitted subtypes using the permits keyword in the sealed class declaration.

Example:

public sealed class Shape permits Circle, Triangle {
    // ...
}

• In this example, only the Circle and Triangle classes can be direct subtypes of the Shape class.
• You can also have a default case to allow other classes within the same package to extend the sealed class.

A permitted subclass may be declared non-sealed, then it is open for extension.

public non-sealed class Shape permits Circle, Triangle {
    // ...
}

A sealed class imposes three important constraints on its permitted subclasses:

All permitted subclasses must belong to the same module as the sealed class.
Every permitted subclass must explicitly extend the sealed class.
Every permitted subclass must define a modifier: final, sealed, or non-sealed.

Can be used for classes and interfaces. But not for record classes or enums?

To be implemented:

• sealed, non-sealed and permits pseudo-keywords used in the class definition context
• important - these are not reserved keywords, they can be used as identifiers (e.g. variable name)
• use isKeyword() token look-ahead instead of string literals in java.g