Allow repeated keys in RandomCollection

[SirYwell]

Overview

Description

Noise-based patterns can benefit from allowing the same pattern multiple times, e.g. #simplex[10][red_wool,blue_wool,red_wool,green_wool] maps random values from [0, 1) to [red_wool, blue_wool, red_wool, green_wool] ([0, 0.25) -> red_wool and [0.5, 0.75) -> red_wool), while before, red_wool was just added up to have a weight of 2.

Additionally, we move the random instance out of the collection. The collection doesn't actually depend on a (stateful) random, so that's a bit cleaner.

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