Accept Docstring for Struct Fields

[julianbuettner]

Motivation

I really missed the feature to add docstrings to fields, especially when using #![warn(missing_docs)] or something alike.

Context

Full disclaimer: the generated changes are llm assisted, but the changes are small enough so that I think they are reviewable.

How to test

cargo new x

use error_set::error_set;

error_set! {
    Foo := {
        Bar {
            /// This is a docstring
            foo: u8,
        }
    }
}

And compare both verions of error_set:

[dependencies]
# comment one!
error_set = "0.9.1"
error_set = { git = "https://github.com/julianbuettner/error_set.git", rev = "12dda7ed37108d74456d89f88667c08b936c54fd" }

Then run cargo doc and open the docs