Add mocking attributes or macros

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:

```rust
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).