add support for Java 17

[monperrus]

No description provided.

[SirYwell]

Overview of new JDK features: https://openjdk.java.net/projects/jdk/17/

Ignoring preview features, following JEPs affect the JLS:

• JEP 306: see changes in the JLS
• JEP 409: see changes in the JLS

I didn't really look into 306 yet, but already tried to make some progress in terms of sealed classes. I already did a basic implementation targetting https://openjdk.java.net/jeps/397, I didn't had the time to make sure it's fully compatible (in terms of the spec and JDT) but I don't expect any major issues there. The issues I came across when working on that are covered by:

• Adding and removing extended modifiers #4136: Marking enums as final/sealed is somewhat complex otherwise
• review: refactor: Update EnumsTest to JUnit 5 #4138: Changes to enums should be tested too, and looking into the enum tests releaved some other issues (partly mentioned in comments too)
• refactor: introduce ModifierTarget for better dealing with modifiers #4177: Useful to make handling of the modifier bits easier and more resilient
• (chore(deps): update jdt to 3.27 #4164: Required to implement it as non-preview feature)

Once I updated my experiments to the new JDT version, I'll open a wip PR for it.

[monperrus]

Thanks Hannes!