MoonSharp / the future of Lua

(going to flesh this out more later the can has been kicked down the road now that Lua works under Mono)

(see existing discussion in https://github.com/TASVideos/BizHawk/issues/971#issuecomment-549660627, #1415, #1422, and #1491, and also in IRC logs if you have them)

options to choose from:

• Lua
  • (status quo) NLua w/ same API surface
  • MoonSharp w/ same API surface (fully managed, but it's still only up to Lua 5.2)
    • or NeoLua, which is similar but with the DLR and, crucially, Lua 5.3 support i.e. integers
  • Pluto (fork of Lua 5.4, probably no C# bindings)
  • Roblox' Luau w/ same API surface (fork of Lua 5.1 with more or fewer features compared to Lua 5.4, notably no integers or stdlib io, also I doubt there are C# bindings)
    • https://github.com/LorettaDevs/Loretta is a managed parser which supports the Luau dialect, need to investigate execution
  • some form of Lua w/ new API surface
• C#
  • Roslyn embedding is heavy but functional, case study: Space Engineers
• I'll make my own C#, with blackjack and hookers!
  • I've considered forking the Roslyn C# compiler to create a new .NET language, but C# is becoming increasingly like my vision with every update. So see above re: embedded C#.
• grug
  • New kid on the block, aims to be as embeddable as Lua without all the syntax footguns
• Kotlin
  • blocked by either: 1. Xamarin merger finally adding JVM compat to .NET 8+ (and then us moving to .NET 8+), or
  • 2. Kotlin compiler getting an MSIL target (unlikely)
• Python (see #1243)
  • Python.NET (working, see ext. tools wiki)
  • IronPython 3 (maybe dead?)
• IronRuby (dead)
• Rune
  • blocked by unfinished C FFI (only Rust hosts can spawn a Rune VM for now)

Start by reading the existing discussions in #971, #1415, #1422, and #1491, then compare the Lua, C#, Python, Kotlin, and Rust options listed here. Done means selecting a scripting direction and documenting the resulting API, runtime, and compatibility plan.