Drop PropTypes requirement, Add option for attribute typing

Features:

• Add option for specifying attribute types so it will automatically be converted from the string to the appropriate type. (Number, Object, Function, ref, etc) #50
• Remove PropTypes requirement
• Ensure preact 10 is supported (all tests run against preact 10 too now)
• Begin convert to TypeScript
  • NOTE: use // @ts-ignore on the line before calling reactToWebComponent() to disable type checking if you're in a TS environment. Typing will be implemented properly before stable release.

Example ignoring TS if you want to start using this alpha release:

customElements.define(
  "react-counter",
  // @ts-ignore
  reactToWebComponent(Counter, React, ReactDOMClient, {...})
);

Bug Fixes:

• None

Other:

• Test improvements / fixed intermittently failing tests
• Update docs to use functional components in the examples