add new features easily
(https://github.com/ruby/irb/pull/500)
* Add nesting level parser for multiple use (indent, prompt, termination check)
* Rewrite RubyLex using NestingParser
* Add nesting parser tests, fix some existing tests
* Add description comment, rename method to NestingParser
* Add comments and tweak code to RubyLex
* Update NestingParser test
* Extract list of ltype tokens to constants