Add FormAssociation mechanism to automatically map proxy values

[nicholasrice]

The FormAssociated base class leverages a proxy element to support browsers that do not support native form-association. Currently most components add a change handler and manually update the proxy element when parallel properties on the custom element change. This logic is largely duplicated.

We should provide a simple way to declare that a property should be reflected to the proxy element so that logic can be encapsulated. In general the mapping is 1-1 but there is 1 case where the value is mapped via the setAttribute API so there will need to be some level of configuration required.

This could look something like:

// before:
@attr
public name: string = ""
private nameChanged() {
  if (this.proxy instanceof HTMLElement) {
    this.proxy.name = this.name
  }
}

// After
@attr
@formAssociatedMapToProxy
public name: string = ""

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[janechu]

Per #6955 this issue is now obsolete.