chore: parse fieldless structures

[ghost]

Description

Problem

Resolves #3018

Summary

allows such programs:

struct Hello;

fn main() {
    let hello = Hello {};
}

but not

struct Hello;

fn main() {
    let hello = Hello;
}

warning: unused variable hello
  ┌─ main.nr:4:9
  │
4 │     let hello = Hello;
  │         ----- unused variable 
  │

error: cannot find `Hello` in this scope 
  ┌─ main.nr:4:17
  │
4 │     let hello = Hello;
  │                 ----- not found in this scope

PR Checklist

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[TomAFrench]

Can you add an example to the struct integration test?