Now the first pass of migrating to TypeScript is complete I'm going
through the src files one by one to tidy up the public/private
interfaces.
Puppeteer used an underscore convention to denote privacy but violates
this in some places; we should be strict with TypeScript's `public` and
`private` keywords instead.
This means we'll get nice TS errors if you try to refer to a private
method/variable, and means when we swap to generating our TS docs the
tooling knows what method(s) are public and therefore need to be
documented vs private internals that don't.
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
