feat: upgrade to React 18 (thebuilder#538)

fix: Update test-utils.ts (thebuilder#525)

feat: add support for `fallbackInView` (thebuilder#521)

fix: undo the clear observers (thebuilder#523)

fix: correct import path for observermap (thebuilder#522)

fix: clear observers after each test run (thebuilder#519)

fix: make ratio return largest threshold smaller than trigger (thebui…

…lder#510)

fix: make children optional (thebuilder#505)

feat: export the observe method (thebuilder#473)

fix: use WeakMap to avoid memory leak (thebuilder#463)

IntersectionObserver roots were tracked in a Map. This ensures it's possible to reuse the Intersection Observer instance if they share the same options. Changed to use a WeakMap, so unused root elements can be garbage collected.

Closes thebuilder#462