Unused variable warning and unused variable notation

A feature I really wish to see in Rhombus is static warning about unused variables, as these are usually mistakes. DrRacket in a sense has this feature already via the tooltip "no bound occurrences". Racket Mode makes this even more explicit by highlighting these unused variables.

However, there are also legitimate unused variables. It would be annoying to see warnings for these legitimate cases. Several languages solve this problem by adding a special notation to indicate unused variables, usually by using identifier _ or identifier prefixed with _. Editors / tools then can recognize these identifiers and don't report them as unused.

Note that this is not just a convention. It needs a support from the language. For example, the following code should work:

(define (foo _ a _ b)
  (+ a b))

but currently Racket fails with:

define: duplicate argument identifier in: _

Similarly, the following code should work:

(define-values (_ a) (values 1 2))
(define-values (b _) (values 1 2))
(+ a b)

but currently Racket fails with:

module: identifier already defined in: _

There's also a question about how to make the notation customizable. It should be possible to create a new language where this unused variable notation is disabled, for example.

The issue names no files or tests. Start by tracing how Rhombus handles function arguments and define-values bindings, then determine how repeated or prefixed unused-variable notation and language-level customization should work; done means the examples compile without duplicate-identifier errors and unused-variable warnings distinguish intentional bindings.