fix: correct parsing of nested sealed classes

[clementdessoude]

What changed with this PR:

Fix parsing of nested sealed and non-sealed classes

Example

// Input
package nl.jqno.equalsverifier.integration.extended_contract;

public class SealedClasses {
    public static sealed abstract class SealedParent permits SealedChild {}

    final static class SealedChild extends SealedParent {}
}

// Output
public class NestedSealedClasses {

  public abstract static sealed class SealedParent permits SealedChild {}

  static final class SealedChild extends SealedParent {}
}

Relative issues or prs:

Fix #522