[Good first issue!] Cleaning up ESLint issues and adding good defaults

[matthew-dean]

At one point, Less was converted from JSHint to ESLint, but ESLint still has some existing warnings.

In addition, there are currently only a few preset defaults set in an .eslintrc.json file in packages/less. The defined rules are okay, but ideally there would be a simple standard which would largely conform to those rules. Another problem with customizing rules is it prevents adding types in TypeScript. Constructs like TypeScript const enums will throw ESLint errors / warnings for no-unused-vars, for example.

So the task would be:

1. Updating ESLint config
2. Finding a good JavaScript & TypeScript recommended standard that are mutually compatible.
3. Removing rules already-defined in those standards.
4. Fixing any remaining issues to have no ESLint warnings / errors in CI.

Ideally, any ESLint changes would absolutely not cause changes in each and every file everywhere. So, no changes should be made that, for example, change the number of indent spaces.

Any takers?

[matthew-dean]

cc @iChenLei

[matthew-dean]

We should maybe consider a more opinionated style set of rules like: https://github.com/xojs/xo

[matthew-dean]

@iChenLei I opened this as a "discussion" instead of an issue because of how our issue flow is set up, but I'm thinking that things like this should probably be "issues" so they can be linked / closed by PRs like: #3790