@Immutable annotation for value classes

Hi!

I am currently migrating a projeect from jsr305 to `java-annotations`. One of the annotations I can't really seem to find an equivalent for is `@Immutable`. A class being `@Immutable` should not have setters and possibly not even non-final fields.

The original implementation and documentation:

```java
/**
* The class to which this annotation is applied is immutable. This means that
* its state cannot be seen to change by callers. Of necessity this means that
* all public fields are final, and that all public final reference fields refer
* to other immutable objects, and that methods do not publish references to any
* internal state which is mutable by implementation even if not by design.
* Immutable objects may still have internal mutable state for purposes of
* performance optimization; some state variables may be lazily computed, so
* long as they are computed from immutable state and that callers cannot tell
* the difference.
*

* Immutable objects are inherently thread-safe; they may be passed between
* threads or published without synchronization.
*/
@Documented
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.CLASS)
public @interface Immutable {
}
```

The issue names no source file or test path; start by reviewing the repository's existing annotation definitions and their conventions. Done means adding an equivalent @Immutable annotation with documented immutability semantics and appropriate type, retention, and documentation metadata.

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