Revamp QA suite

[oxyc]

A lot of the QA suite was experimentations on my part, but I think it could be loosened now.

• Drop support (or at least testing) for ancient JavaScript engines.
• Drop complexity analysis. Or just print it without failing.
• I think we should still keep test coverage analysis and fail if there's a reduction.
• Maybe drop jshint and adopt prettier so contributors don't need to care about code standards. To me which convention doesn't matter as long as there is one.
• Should make it easier and better documented how to scaffold tests.
• I have nothing against spec as a testing framework but if we drop support for ancient JavaScript engines this is also open for discussion. Eg. ava which is async. But as spec works and is fast enough imo, I don't see much worth changing it.
• Definitely drop testem, unless it's easy to add browser testing I think node testing would be enough for luaparse.
• Is it possible to drop the UMD wrapper and use some library that wraps everything during the build step? Luaparse shouldn't be keeping it up to date.

Thoughts?

[ghost]

Should make it easier and better documented how to scaffold tests.

Really this will help me a lot to pull request again...