derive: snippet support

[zkat]

The current derive macro supports code, help, and derive, but doesn't really handle the snippet part of miette. Extend the macro so that you can write stuff like this:

#[derive(Debug, Error, Diagnostic)]
#[diagnostic(code(math::bad_arithmetic))]
#[error("Tried to add a {bad_type} to a {good_type}")]
pub struct MyErr {
  good_type: Type,
  bad_type: Type,
  bad_var: Var,
  src: PathBuf,

  // The overall span of code that will be rendered.
  #[context(src, "This region is where things went wrong")]
  ctx: SourceSpan,

  // A highlight (basically something to underline)
  #[highlight(ctx, "This is a {bad_type}")]
  bad_var_span: SourceSpan,

  // Can have multiple highlights.
  #[highlight(ctx, "This is a {good_type}")]
  good_var_span: SourceSpan,
}