This change enforces how we type arrays, e.g. choosing between:
* `string[]`
* `Array<string>`
I've gone for the `array-simple` option [1] which enforces that:
* primitive types and type references use `X[]`
* complex types use `Array<X>`
For example, we'd type an array of strings as `string[]`, but an array
of a union type as `Array<SomeUnionType>`.
[1]: https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/array-type.md
