Custom luakittens parser

[Dich0tomy]

As per #7:

This implementation is incomplete and I will be rewriting it to a handwritten parser, which will allow for more fine-grained syntax control and better diagnostics.

We need to support:

• alternative types properly
• alternative types properly
• type grouping ([](string|number)?)
• properly support aliasing multiple returns and such (when functions will become a thing), but also for aliases/strong types if they will be supported
• rewrite nils and alternatives to support them at grammar level but not typesystem level (AST validation & rewrite)
• do we also make any actually be any type but not nil? And then only any? or any|nil would work as the classic any? Put that behind an option? lose_any or something?