fix: bind unexported error sentinels as error

[ivov]

Bindgen now emits error for exported vars whose Go type is an unexported named type satisfying error. The most visible case is cmpopts.AnyError, whose underlying anyError is struct{} and previously surfaced as Lisette's unit type (), leaving the var unusable without a manual cast.

import "go:errors"
import "go:github.com/google/go-cmp/cmp"
import "go:github.com/google/go-cmp/cmp/cmpopts"

let err: error = errors.New("boom")
cmp.Equal(err, cmpopts.AnyError, cmpopts.EquateErrors()) // true

[github-actions]

Released in lisette-v0.1.26