Update to EsLint v6

[EnTeQuAk]

WIP PR: #2652

[eight04]

I got some reports from npm audit:

                       === npm audit security report ===


                                 Manual Review
             Some vulnerabilities require your attention to resolve

          Visit https://go.npm.me/audit-guide for additional guidance


  Moderate        Regular Expression Denial of Service

  Package         acorn

  Patched in      >=7.1.1

  Dependency of   web-ext [dev]

  Path            web-ext > addons-linter > eslint > espree > acorn

  More info       https://npmjs.com/advisories/1488


  Moderate        Regular Expression Denial of Service

  Package         acorn

  Patched in      >=7.1.1

  Dependency of   web-ext [dev]

  Path            web-ext > addons-linter > eslint-plugin-no-unsafe-innerhtml
                  > eslint > espree > acorn

  More info       https://npmjs.com/advisories/1488

found 2 moderate severity vulnerabilities in 4008 scanned packages
  2 vulnerabilities require manual review. See the full report for details.

Does upgrading eslint solve the problem?

[EnTeQuAk]

@rpl @willdurand I'm not able to finish this off. I don't think it's urgent but unfortunately it has been sitting around for a long time. On the other hand, from what I figured with playing around and wasting lots of time in the PR is that the eslint APIs have changed significantly again and especially mocking has become harder. I hope the work in the PR is useful.

[willdurand]

done.