groupByMultiple(
[{ type: 'a', color: 'red' }],
['type', 'color']
)groupByMultiple creates nested groups by applying multiple discriminator keys sequentially.
Challenge: Implement groupByMultiple<T extends object>(arr: T[], keys: (keyof T)[]): Record<string, T[]> creating a compound group key from multiple fields.
groupByMultiple([{a:1,b:'x'},{a:1,b:'y'},{a:2,b:'x'}], ['a','b'])
// { '1-x': [...], '1-y': [...], '2-x': [...] }