Add initial support for Pluto

[Sainan]

This adds --features pluto, relevant methods on LuaVersion, and implements parsing for many of the syntax additions:

• Compound operators
• Member access safe navigation
• Continue statement
• Numeral parsing
• Universal block opener
• Operator aliases
• Counted break statement
• Unicode identifiers
• Coalescing operator
• If expressions
• Walrus operator

I would like for this to be a complete PR, but the language has far too many syntax additions for that. Features that are not implemented here yet include:

• Destructuring (local { a } = { a = 1 } and local [ a ] = { 1 })
• Ternary expressions (a ? b : c)
• Type hinting (roughly a subset of Luau's from what I can tell)
• Lambda syntax (|a| -> tostring(a) and || -> do print("hi") end)
• Pluto's string interpolation (e.g. $"1 + 2 = {1 + 2}")

[Sainan]

I think at this point the PR is good for review so I at least know if this is the right direction and if I understood the project structure correctly. I'd be happy to keep making subsequent PRs to move progress forwards on this.