Introduce xoshiro RNG to generate dungeon seeds

[StephenCWills]

Implements the technique for mitigating overlapping random number sequences described in #6438 (comment).

Summary of changes:

• Introduce the xoshiro128++ RNG
• Rename GenerateSeed() to GenerateRandomNumber()
• Use xoshiro128++ to reimplement GenerateSeed()
• Rename GameData::dwSeed to GameData::gameSeed
• Use 64 bits for the game seed and use std::chrono to generate it
• Update logic for generating dungeon seeds using xoshiro128++ instead of LCG

The 32-bit overload for xoshiro128plusplus::seed() is intended to provide a way to reseed xoshiro128++ using a dungeon seed. This will become necessary to create a deterministic sequence of seeds for monsters, objects, and items during dungeon generation. It feels a bit like falling back on old habits to use this random number generator to seed itself, but the SplitMix32 implementation makes it work.

This replaces #6438
This resolves #6261

[ephphatha]

I like this implementation (I was looking at using xoshiro128** myself but couldn't think of a good initial use case :D), a few nits and the only real concern is whether we want to give the option of using the old dungeon table logic for people who want to stay close to the original game.

Regarding "It feels a bit like falling back on old habits to use this random number generator to seed itself, but the SplitMix32 implementation makes it work." the flaw with the old logic is that it's using a single set of parameters for the LCG to seed itself, so all sequences are overlapping. This implementation solves that since you're using three different families of function (SplitMix > xoshiro > LCG) to seed each other.

You could even use the global xoshiro128plusplus generator to spawn instanced xoshiro128plusplus generators by making use of the jump() function, the purpose of that is to advance the state far enough that the sequences will not overlap in their usage lifetime.

Real excited about potential future improvements too. It would be amazing to see instances of xoshiro128++ used for monster AI, that'd let multiplayer games get away from constantly reseeding the global RNG in an attempt to sync monster behaviour.

[ephphatha]

RNG implementation looks good to me, if we find a platform that implements std::random_device as a SplitMix64 generator seeded with the current unix timestamp then I guess we gotta revisit how the global rng gets seeded :D

[StephenCWills]

Assuming this PR makes it into 1.6, don't forget to merge diasurgical/devilutionx-gamelist#27 and update the Discord bot at some point before the release.

[AJenbo]

Scary stuff, lets hope this works out as planned :)