Implement a parser

[vurvdev]

Need this for #5

[vurvdev]

Going to be doing this with some sort of JS parsing library. Peggy seems like the way to go, since Nearley is getting abandoned..

In the future the parser should be shared with https://github.com/Vurv78/ExpressionScript for parity but they can be separate at first (Bridging the gap between a Haxe project and Typescript a project would be awkward as well)