Implement ES2024 `Map.groupBy`

[robik]

Summary

This PR implements EcmaScript 2024 Map.groupBy

Test Plan

Added tests.

>> Map.groupBy([1, 2, 3, 4, 5, 6, 7], function(key) { return key % 2; })
Map { "1" => [ 1, 3, 5, 7, [length]: 4 ], "0" => [ 2, 4, 6, [length]: 3 ] }

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