Export Formatter, Unit and Suffix types

[amorey]

Fixes #230

This PR exports Formatter, Unit and Suffix types which were present in v7. Now you can do this again:

import type { Formatter, Suffix, Unit } from 'react-timeago';

const myCustomFormatter: Formatter = (
  value: number,
  unit: Unit,
  suffix: Suffix,
  epochMilliseconds: number,
  nextFormatter: Formatter,
  now: () => number,
) => {
  if (suffix === 'from now' || unit === 'second') return 'just now';
  if (nextFormatter) return nextFormatter(value, unit, suffix, epochMilliseconds, nextFormatter, now);
  return '';
};