Mutation observers: Avoid adding disconnected elements

[javan]

This change works around a somewhat unexpected MutationObserver behavior: When a node is removed from an observed tree and then another node is added to it synchronously, the observer will include that added node in its next batch of mutation records. Example: https://jsfiddle.net/javan/acxd6v1x/

So, if you remove an element from the DOM and then add a data-controller element to it, Stimulus will match the added element and install a new controller. Because the element is already disconnected from the DOM, Stimulus will never unmatch and disconnect() its controller. AKA a memory leak. Example: https://jsfiddle.net/javan/vqht4n5s/

[sstephenson]

Do we need an isConnected polyfill?

[javan]

The isConnected test is an optimization.
https://github.com/stimulusjs/stimulus/blob/d3ea33716e2315ae3a1c0189e61bb1e136eccd82/packages/%40stimulus/mutation-observers/src/element_observer.ts#L132-L136
Browsers that do support it will return a boolean, and if there's a mismatch we can skip the slightly more expensive element.contains() call in the else branch. Browsers that don't support will return undefined and jump straight to the else branch.

[javan]

A polyfill might look something like this, which would end up being even more work:

if (!("isConnected" in Node.prototype)) {
  Object.defineProperty(Node.prototype, "isConnected", {
    get() {
      return document.documentElement.contains(this)
    }
  })
}

(not that it really matters)