Add support for @CheckReturnValue

[cescoffier]

IntelliJ has a "clever" way to inform the users that the result of a method must be used.
It detects that the method is annotated with a @CheckReturnValue annotation regardless of its package. So, frameworks can provide their own (like SmallRye Commons Annotations).

Basically, if you have an API with:

@CheckReturnValue
public Uni<String> doSomethingCool();

a user using that method like in the following snippet would get a warning:

doSomethingCool();

The warning indicates that the return value must be used, like in:

doSomethingCool().subscribe().with(r -> ...);

This is particularly interesting for reactive APIs, stream APIs, and builders APIs.

It would be great if VS Code Java could provide the same kind of feature.

CC @maxandersen