feat: zero-fill spread

[ivov]

Adds zero-fill spread syntax .. to fill unspecified struct fields with their zero value.

let cmd = cobra.Command {
  Use: "greet",
  Run: |_, args| { fmt.Println("hi") },
  ..
}

let p = Point { x: 10, .. }   // y: 0
let q = Point { .. }          // x: 0, y: 0

Zero-fill spread works for Lisette structs, Go-imported structs, and for enum struct variants.

• For Go-imported types, .. always works.
• For Lisette types, .. is allowed when every unspecified field has a defined zero (primitives, Slice, Map, Option, tuples, nested zero-fillable structs). Types without a zero (Ref<T>, Channel<T>, bare function types, Result<T, E>) must be provided explicitly.

[github-actions]

Released in lisette-v0.1.22