Uglify 3 merge

[mjpieters]

Merge in Uglify master and modernize

This brings this codebase up to speed with current JS standards.

• Moved tests to an updated version in test/feature_extractor.js (based on test/compress.js, which in turn was evolved from test/run-tests.js)
• Update test/feature_extractor/*.js files to use valid JS syntax (multiline strings in JS template strings)
• Drop use of deprecated sys methods (using console instead).

Upstream changes supporting the development process (.github/ and CONTRIBUTING.md) were not included in this merge.