feat: Add unit literals and unit types to parser

[jfecher]

Description

Problem*

Resolves #1933 and #1937, succeeds #1956

Summary*

Previously all parenthesized types were parsed as tuple types. I've fixed this to have:

• parenthesized singular types (with no trailing comma) parsed as the parenthesized type. So (Field) = Field.
• tuple types with 0 elements are always parsed as Type::Unit.

Documentation

• This PR requires documentation updates when merged.

  • I will submit a noir-lang/docs PR.
  • I will request for and support Dev Rel's help in documenting this PR.

Additional Context

PR Checklist*

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

[ghost]

don't forget about this case:

fn test() {
    ()
}

[jfecher]

Right, this is also somewhat of a feature addition since we didn't have unit literals at all anymore. I think I'll rename the PR to reflect this.