Migrate linting from standard to XO

[jfmengels]

Migrate linting from standard to XO, to have a consistent linter and style accross the Zeit projects, as requested by @leo.

Notes:

• I added a comment to disable ava/no-ignored-test-files. It's a rule that warns you against using ava in files that will not be run by AVA (because they are not in the default or custom glob, which in this case is "build/test/*.js").

You could:

• Leave the comment as is, or disable the rule altogether (on such a small project, that's probably the way to go as the test process is already set up)
• Or modify the AVA config so that it transpiles your code like the following:

{
  "...": "...",
  "ava": {
    "...": "...",
    "babel": {
      "babelrc": true, // If you have a babelrc file
      "presets": ["es2015", "stage-2"] // along with plugins, if you don't
    },
    "require": [ // Similar to `ava --require babel-register`
      "babel-register"
    ]
  }
}

That way, you get:

• the safety of that eslint rule
• You don't have to build your tests (if that makes things easier)
• You may get better error reports, with regards to stacktraces and reports of where the error comes from

[leo]

It's ok like that, thanks! 😊