Convert observer utility components to TypeScript

[pugnascotia]

Summary

Convert the observer utility components to TypeScript. I added a dependency on @types/resize-observer-browser since ResizeObserver doesn't appear to be supported in TypeScript yet.

I checked that the examples in the docs site still worked in Chrome.

I also fixed a bug where the prop-types Babel plugin would bail on components with no props e.g.

const Foo: FunctionComponent = props => { ... }

Checklist

• This was checked in mobile
• This was checked in IE11
• This was checked in dark mode
• Any props added have proper autodocs
• Documentation examples were added
• A changelog entry exists and is marked appropriately
• This was checked for breaking changes and labeled appropriately
• Jest tests were updated or added to match the most common scenarios
• This was checked against keyboard-only and screenreader scenarios
• This required updates to Framer X components

[chandlerprall]

LGTM! Pulled and built TS defs locally, confirmed the observers are still exported as expected.