Create our own Random class using the xoshiro256 algorithm

[initsu]

I did not find a library that satisfied all requirements, so I just made a variant of existing open source (MIT or Public Domain) code.

• This same algorithm can be chosen by .NET internally (and is Public Domain), but there is no guarantee that this is the case. By having a specific implementation, we can guarantee deterministic output.
• This gives access to the state of the Random object at any point via GetState and SetState methods that use Base64 strings. This should allow us to capture the RNG state if an error occurs and reproduce it as needed.
• This Random class exists in the RandomizerCore package and will take precedence when working inside this package. It is not a subclass of the System.Random class, so trying to use the wrong class somewhere will just not compile. It has no constructor without a seed. When we don't need to specify a seed, we can still use System.Random (as that implies we don't care about deterministic output).

Implements #395

[Ellendar]

Not going to merge this for 5.1 . Probably just adding it wont cause issues but actually using it is not something I want to introduce when i'm trying to finish up and be ready to ship 5.1. Feel free to start up a branch for 5.2 and we can merge it there.