fix: use WeakMap to avoid memory leak (#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 #462

Author: thebuilder

src/observers.ts

@@ -9,7 +9,7 @@ const ObserverMap = new Map<
   }
 >();
 
-const RootIds: Map<Element, string> = new Map();
+const RootIds: WeakMap<Element, string> = new WeakMap();
 
 let rootId = 0;