Implement temporary alternative IDE-compatible annotation for "value" keyword

Early experimenters with with value types are finding
that the "codes like a class" aspect works well.

One difficulty is that the "value" keyword confuses
IDEs and editors.

If the javac parser were to accept "@__value__"
as a *temporary* alias for the "value" quasi-keyword,
IDE and editors would be able to process the syntax
more easily.

It's a shallow change, and one we'd want to remove
as IDEs adopt the "value" keyword, but it might help
reduce friction for early experimenters.