scan: tokens representing special terminals can be constants
test: remove test_dev.html. Same capabilities exist in src/js/test/test.html.
api+scan: remove debug-only tokenize(). Clients can simulate it.
scan: make r5js.Scanner state private where possible.
scan: use a more OO pattern for keeping instance state
scan: move r5js.Scanner under src/js/scan and run through linter