Add mocking attributes or macros

[bitwizeshift]

This is a larger task that will make use of #11 in order to generate/define mock functionality.

The exact syntax/specifics are yet to be determined, but the idea is to support something like:

trait Turtle {
    fn pen_down(&mut self);
    fn pen_up(&mut self);
}

#[neotest::mock]
struct MockTurtle;

impl Turtle for MockTurtle {
    // Set this as a mock function
    #[neotest::mock_fn]
    fn pen_down(&mut self);
    #[neotest::mock_fn]
    fn pen_up(&mut self);
}

// ...

fn test_draw_circle() {
    let mut turtle = MockTurtle::new();
    let painter = Painter::new(&mut turtle);

    subtest!{ |calls_pen_down| 
        let expectation = neotest::expect_call!( turtle.pen_down() => Times(1) );

        painter.draw_circle(0, 0, 10);
        
        assert!(expectation.satisfied());
    }    
}

(Syntax/names are subject to change, but this is the general idea).