fix(groupBy): Use more accurate return type

[the5thbeatle]

Summary

Updates the return type of groupBy to reflect that properties need not exist in the output but that if they do, their value has at least length 1. It prevents users from having to suppress unnecessary condition warnings when trying to safely access properties in groupBy results.

Changes

• Wraps groupBy return type with Partial
• Changes values in groupBy return type from T[] to [T, ...T[]]

Comments

The compat function is still using Record type here instead of Partial<Record>