refactor: simplify DOM, event, and observer module surfaces

[jderochervlk]

Summary

• add concrete-module t aliases across the DOM, event, UIEvents,
  and observer surface
• update DOM-facing tests to use Window.current and concrete module
  aliases instead of reaching into raw *Types modules
• remove the trivial DOM
  voidFunction alias while keeping meaningful callback contracts such as
  frameRequestCallback

Validation

• npm run build
• npm test

Related

• Refs Simplify DOM, event, and observer module surfaces #238