Add `const` and `unsafe` block suport to `bsn!` macro (#24336)

# Objective

Closes #24121

## Solution

Add `const`, `unsafe` and `async` block support to bsn expressions.

## Testing

I've added a compile test for `const` and `unsafe` blocks. I couldn't
figure out how to test or even use the async blocks. The macro needs to
construct a struct that holds a future, but `async` blocks produce
voldemort (unnameable) types. Using `Box<dyn Future>` fixes this problem
but then you run into the problem that the struct needs to be clone-able
and have a default value. `async` block futures have neither of those
properties.

---

## Showcase

The `bsn!` macro now supports `const` and `unsafe` blocks.
```rust,
 fn friendly(people: &[&'static str]) -> impl Scene {
     bsn! {
         Friend {
             name: const {"John"}
             // SAFETY: Jesus take the wheel
             father: unsafe {people.get_unchecked(0)}
         }
     }
 }
 ```