Fluent and L10nRegistry code is currently linted according to https://github.com/projectfluent/fluent.js rules. We should align it with https://dxr.mozilla.org/mozilla-central/source/tools/lint/eslint/modules.json and enable linting.

The basic alignment happened in https://github.com/projectfluent/fluent.js/commit/8e4f284e04b9333393da8c66dca46d82f3e60b57 With this in place our code conforms to the mozilla rules. We can look deeper into reusing mozilla's eslint ruleset, but that's not a blocker anymore and would rather be a good base for fluent.js repo eslint refresh.