Spread operator (`...`) for arrays and dicts

[harehare]

Description

Add a spread operator (...) to the mq language for expanding arrays and dicts inline within array literals and dict literals.

Proposed syntax

Array spread

let a = [1, 2, 3];
let b = [4, 5, 6];
let c = [...a, ...b];      # [1, 2, 3, 4, 5, 6]
let d = [0, ...a, 99];    # [0, 1, 2, 3, 99]

Dict spread

let base = {x: 1, y: 2};
let merged = {...base, y: 99, z: 3};   # {x: 1, y: 99, z: 3}

Later keys override earlier ones when the same key appears more than once.