Add Support for Command Line Flags at Language Level

[JasonSteving99]

Needs both research into implementing this with a good flags library, and also blocked by Javac Multi Files https://github.com/JasonSteving99/claro-lang/projects/1#card-59558893

Claro's going to be used in production.. let's just acknowledge that now and build in common sense things that are gonna be needed in any production system. Let's start with flags.

e.g.:

final var ENV: string = Flags.string("env", "flag description"); final var OPTIONAL_BEHAVIOR: optional = Flags.optionalInt("optional_behavior", "flag description");