Support default field values

[DJMcNab]

This uses my fork of syn defined at dtolnay/syn#1870, and is intended to validate that. This PR is not necessarily intended to merged/reviewed, as things stand. In particular, if the syn PR is never merged, this won't be the right solution.

• RFC: [RFC] Default field values rust-lang/rfcs#3681
• Tracking issue: Tracking issue for RFC 3681: Default field values rust-lang/rust#132162
• Feature gate: #![feature(default_field_values)]

#[derive(Parser)]
struct Opt {
    arg: i32 = 42,
    //       ^^^^
}

Some notes:

• Supporting default field values is frustratingly involved, because of how the support has to be hacked into syn 2.0. See the suggestion in Support parsing #![feature(default_field_values)] dtolnay/syn#1851 (comment) of the only way to move this forward. This will need to filter through the ecosystem if approved, which this PR is part of.
• Testing the use of this feature in CI is hard because it requires nightly
• There are some pretty nasty hacks in here; obviously those can be cleaned up before landing.

The test can be ran with:

RUSTFLAGS="--cfg=nightly" cargo +nightly test --test default_field_values --features derive --features help --features usage

[DJMcNab]

Closing in favour of #6220