feat: Add cellToChildren(parent, resolution)

[donmccurdy]

Adds a new utility function, which returns a list of all child cells at a given resolution within a parent cell.

Usage:

const parent = 5224972163924099071n; // res=8
const cells = cellToChildren(parent, 9);
// → 5229475712011862015n, 5229475729191731199n, 5229475746371600383n, 5229475763551469567n

Note that geometryToCells(cellToBoundary(parent), resolution) will not give the same result, as the conversion to/from a polygon may pick up extra cells at the boundaries due to floating point precision.

[donmccurdy]

CI should pass for this PR after #17.