tools: enable all es6 rules in linter
This change enables the es6 environment in eslint. Rather than selecting es6 rules one at a time, this turns all of them on with the exception of modules.
Ref: https://github.com/nodejs/node/pull/5028
Not sure if this will be controversial or not. And not sure if it makes sense to land on LTS or not. (Maybe we shouldn't use es6-isms not supported by LTS? Certainly not for code that needs to land on LTS, of course. But I'm not sure a lot of these rules will actually cover things that don't work in LTS but work in Stable. Will have to look into that...)