Pull in the parse helper to lexer_spec as chainable method
Move ParserHelper methods to a module in its own file
Move each section of parser spec to its own file
Add build_lexer to test task dependency
Import changes to lexer_skeleton from "upstream" work by @YorickPeterse.
See https://github.com/YorickPeterse/oga/commit/54e6650338e97657e9611320bd0c092428592125