[TS] Show fn optional arg and export interfaces

[Grohden]

Just type fixes

• show funciton arg is said to be optional on readme
• Export interfaces to be able to create type safe objects:

  
  const unsafeProps = {
  friction: '10' // no problemo
  }

const safeProps: SlidingUpPanelProps = { friction: '10' // error, must be number }