Add shadowRootOptions for more easily customizing shadowRoot

[sorvell]

Description

For forward compatibility will the next version of LitElement, static shadowRootOptions should be added. This is a slightly easier way to customize the renderRoot.

Instead of:

createRenderRoot() {
  return this.attachShadow({mode: 'open', delegatesFocus: true});
}

users can just do:

static shadowRootOptions: {mode: 'open', delegatesFocus: true};

Acceptance criteria

PR landed with feature.