Utils: Add a flatten function to Utils.py

[nex3]

What is this fixing or adding?

This function is useful for #3128 and may be useful for other
multiworlds. SM uses it as well, but only for the varia randomizer
that's pulled in from elsewhere so it doesn't make sense to re-use it
there.

How was this tested?

This uses essentially the same implementation as the function already
in the SM varia randomizer.

[Berserker66]

Isn't this case already covered by the standard lib?
https://docs.python.org/3/library/itertools.html#itertools.chain

[beauxq]

A difference from itertools.chain is that this would flatten an arbitrary number of levels of lists.
But I'm not sure that's a good thing to support.

If a developer doesn't know statically how many levels deep their data structure is, that doesn't seem like a good pattern.
I would suggest that it should be refactored so that there is a known number of levels, so that itertools.chain or something more specific can be used.

Notice that the type annotations are not correct for arbitrary numbers of levels. It's not even clear to me how many levels it would have to be to get it right.

[nex3]

There are two usability benefits here versus itertools.chain() that make this particularly well-suited for writing large chunks of data by hand (as you do when defining information for Archipelago games):

1. It returns a list, rather than an iterator, so it can be more easily used without having to do additional generator syntax on top.

2. It supports mixed values and iterables, so you can have a list that occasionally has sublists. I use this in my DS3 world updates to more easily generate "bundles" of items with different counts.

The fact that it supports arbitrarily nested iterables isn't important to me, so if that's an issue I can update it to support only one level of iterables as well as direct values.

[beauxq]

I think this would be a better to do this: nex3#13

Even if you don't like that, I still think this is too specific for Utils.py