infrastructure: move test/check-complexity to ESLint complexity check

[ljqx]

https://github.com/jquery/esprima/blob/master/test/check-complexity.js calculates cyclomatic complexity for every function, and prints out the most complex 6 functions.

If any function has complexity > 24, the test would fail.

There are several drawbacks here:

• the tool check-complexity.js relies on, https://github.com/philbooth/escomplex-js, is out of maintenance. All the siblings (forks https://github.com/jared-stilwell/escomplex-js, origins https://github.com/escomplex/escomplex) are out of maintenance too. It doesn't support ES6 (as it depends on esprima 1.x), which means we cannot generate ES6 target if we stick on it.
• It checks the generated output js instead of the source.
• the work done is mostly a lint thing.

As ESLint already natively supports limiting complexity, better to move to ESLint, which is already used in this repo.