Convert to a tree-sitter grammar

[tobiasvl]

Atom is currently beta testing tree-sitter grammars. tree-sitter/tree-sitter is a tool that builds complete parse trees of source code.

Using this instead of TextMate-style regex parsing will make things like #2 and #6 easier to solve.

Support for a tree-sitter grammar can be added alongside the current grammar, so I've started work on it: https://github.com/tobiasvl/tree-sitter-rgbasm