Treat noChange and nothing as any type in binding

[rictic]

The noChange and nothing values have special meaning in a lit binding, and they are legal to use everywhere, as they directly specify behavior rather than an ordinary value to be set/rendered

[WickyNilliams]

related #207

outside of custom directives, you can also trigger analyzer errors using built-ins like the when directive:

html`
  <button
    tabindex=${when(someCondition, () => "-1", () => nothing)}
  ></button>
`