Add reduceChildren iterator

[skirtles-code]

This PR adds a new reduceChildren iterator function. It is intended to be similar to the native Array.prototype.reduce.

Example usage:

import { reduceChildren } from '@skirtle/vue-vnode-utils'

// Inside a render function
const children = slots.default?.() ?? []
const count = reduceChildren(children, sum => sum + 1, 0)

Unlike Array.prototype.reduce, the initial value is a required argument. Making it optional would have been difficult, as reduceChildren also needs to support passing the IterationOptions. This could be revisited if use cases emerge.

This feature should be included in 0.2.0.