More uses of `Object.groupBy` — `countBy`, `partition`, simpler version of `chunk`

[lionel-rowe]

function chunk(input, size) {
  return Object.values(
    Object.groupBy(input, (_, i) => Math.floor(i / size))
  );
}

function countBy(input, fn) {
  return Object.fromEntries(
    Object.entries(Object.groupBy(input, fn)).map(([k, v]) => [k, v.length])
  );
}

function partition(input, fn) {
  const results = Object.groupBy(input, (x) => Boolean(fn(x)))
  return [results.true ?? [], results.false ?? []]
}