We used to use babel as our parser for eslint, because we used
JavaScript language features which the eslint default parser (`espree`)
didn't support.

As `espree` is way faster in parsing JS files, and supports all the
language features we currently use, this seems like a reasonable step
to take. To put numbers to this claim:

1. GitLab UI, `parseForESLint`: 5.89s => 2.00s
2. GitLab, `parseForESLint`: 3:45 min => 0:50 min

In order to "emulate" the capabilities of babel, we also set
`parserOptions.ecmaVersion = 'latest'`. This means with future eslint /
`espree` updates, we will automatically gain support for parsing newer
syntax. Theoretically it could mean, that the supported JS features of
babel and eslint diverge in our projects, but that may not be too much
of a concern, as:
1. `@babel/eslint-parser` might be able to parse things which we do not
   support in our "compilation chain" anyhow
2. It is a two-way-door decision because a project could always choose
   to just switch out the parsers if need-be.

That being said, we made not changes to the assumed global `env`, and
still stick with `es2015`. You can use a newer env if need-be.

In order to use `@babel/eslint-parser` again in your project, just
install it as a dependency and add the following to your eslint config:

```yaml
# If you use `vue-eslint-parser`
parserOptions:
  parser: '@babel/eslint-parser'
# If you do not use `vue-eslint-parser`
parser: '@babel/eslint-parser'
```

BREAKING CHANGE: Switch parser from `@babel/eslint-parser` to `espree`