Write PGN parser and corresponding API

[dignissimus]

TODO

• [x] Define the basic API
• [ ] Write the parser
• [ ] Tests
  • [ ] API tests
  • [ ] Parsing tests

Design questions

• [ ] Should there be a separate ChildNode class?
  • The only difference is the existence of the T data attribute which could simply be T? data inside an all-encompassing Node class. Would merging these two classes make the API simpler to use and easier to manage? Would it be a beneficial change?
• [ ] Should lists be nullable or just allowed to be empty?
• [ ] Do we need a walk function given that we have a transform function?

Parser ideas

• With the parser, I'm considering reducing the use of regular expressions for some of the more complicated patterns and implementing a recursive descent style parser.

---

Work in progress. Comments preceded by Comment are prompts for review comments and dialogue about the design of the API. I am also open to comments about the general Dart coding style. I have included several comments about the design of the API and I am actively looking for feedback.

Resolves #1

[dignissimus]

Happy for this to be closed in light of #3 being opened