Merge Sylvan's parser into Verona

[rengolin]

@sylvanc has a parser in a local repo that can parse Verona. It has a PEG definition that is flexible and the code is reasonably straightforward, both which are great for early prototyping. We should merge this in the src directory and use the generated AST to start lowering MLIR.

We may want to build a robust parser later, with @davidchisnall's recommendations in #102, but that'll be for another ticket.

[mjp41]

Closed by #116