Type support checklist ✅

The way ERT evaluates complex types and language specific features is largely a manual process. We have to manually tell ERT how to interpret things like `extends` and `React.FC

`, so inevitably there will be things that we don't support or haven't considered.

This issue aims to track what features we do and don't support and in cases where a feature is unsupported, we hope to provide a suitable bail-out, similar to what is suggested here #141

If there's something missing on this list, please leave a comment below 😄

## TypeScript
- primitive types `boolean, array, string, number` ✅
- `any` ✅
- `unknown` ✅
- `void` ✅
- functions ✅
- `array[]` & `Array<>` ✅
- interfaces ✅
- types `type foo = string` ✅
- union types `'foo' | 'bar'` ✅
- intersection types `Foo & Bar` ✅
- extends ✅
- keyof ❌
- as ❌
- [template literal types](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-4-1.html#template-literal-types) ❌
- [index signatures](https://www.typescriptlang.org/docs/handbook/advanced-types.html#index-types-and-index-signatures) ❌
- [conditional types](https://www.typescriptlang.org/docs/handbook/advanced-types.html#conditional-types) ❌

## [TypeScript utility types](https://www.typescriptlang.org/docs/handbook/utility-types.html)
- `Omit<>` ❌
- `Partial<>` ❌
- `Record<>` ❌
- `Pick<>` ❌
- `ReturnType<>` ❌

## [TypeScript via @types/react](https://www.npmjs.com/package/@types/react)
- `React.FC` ✅
- `React.memo` ✅
- `React.forwardRef` ✅

## Flow
TBD

## Related issues
#59
#141