Add support for unknown to array-type rule.

[Retsam]

PR checklist

• Addresses an existing issue: unknown support for 'array-type' #4085
• New feature, bugfix, or enhancement
  • Includes tests
• Documentation update

Overview of change:

Currently, when used with TS 3.0, the array-simple rule will error on unknown[], instead enforcing that it should be written as Array<unknown>. A small change was needed for unknown to be considered a simple type with TS 3.0.

Backwards and forwards compatibility

This change works with both Typescript 2.x and 3.x.

The tests are backwards compatible with TS 2.x, because in unknown is already treated as a simple type, in that case. (It's treated as a non-generic type reference)

The implementation is backwards compatible with TS 2.x, because ts.SyntaxKind.UnknownKeyword: will evaluate to undefined: and so that case of the switch statement will just never match with nodeType.kind, which should never be undefined.

CHANGELOG.md entry:

[enhancement] array-type rule handles Typescript 3.0's unknown type properly

[palantirtech]

Thanks for your interest in palantir/tslint, @Retsam! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.

[JoshuaKGoldberg]

👋 @Retsam - I just merged a similar PR that didn't have all the Prettier noise. You should still get the "contributor" badge for you work on this one so I'll clean up the conflicts here and try to merge it in. Thanks for the PR!

[JoshuaKGoldberg]

An extra testling line per test file post-merge - merging in anyway because both contributions were valid. 😊