feat: makes Controller.isConnected an observable property

[nicholasrice]

Description

It is currently very difficult to determine when exactly an element is added or removed from the DOM. Custom elements do have connectedCallback and disconnectedCallback but these are really only useful to the class implementation and not external code. There is no connected or disconnected events emitted, and it is using MutationObserver for this case is very difficult / expensive.

whatwg/dom#533 tracks a proposal to patch MutationObserver with connected and disconnected records, but this feature has not made its way into implementation yet.

We can make "connectedness" easy to observe for FASTElement instances though, simply by making the existing Controller.isConnected property observable.

Motivation & context

What does this get us?

• House-keeping of code relative to, but not implemented in, a FASTElement.
  • adding and removing subscriptions and event listeners, cleaning up element caches, etc.
• Dependency Injection value reconciliation, where DI container is based on DOM location.

There are some additional uses cases outlined in whatwg/dom#533.

Issue type checklist

• Chore: A change that does not impact distributed packages.
• Bug fix: A change that fixes an issue, link to the issue above.
• New feature: A change that adds functionality.

Is this a breaking change?

• This change causes current functionality to break.

Adding or modifying component(s) in @microsoft/fast-components checklist

• I have added a new component
• I have modified an existing component
• I have updated the definition file
• I have updated the configuration file

Process & policy checklist

• I have added tests for my changes.
• I have tested my changes.
• I have updated the project documentation to reflect my changes.
• I have read the CONTRIBUTING documentation and followed the standards for this project.