[TODO] Clean up ESLint issues and improve code quality

[matthew-dean]

I'm not sure how much this is still the case, but when the switch was made to ESLint, a lot of ESLint warnings were not addressed. In some cases, the warnings may have been suppressed, but more ideally would be to adopt something like Standard JS, with maybe a few adjustments / accommodations made for Less.js's current code style so that there isn't a change for every single line of code.