Warn on unused CSS selectors

[Rich-Harris]

With cascade: false, we should be able to determine whether or not a CSS selector is used:

<div class='foo'>this is a div</div>
<span class='{{class}}'>this is a span with a dynamic class</span>

<style>
  /* keep these */
  * {...}
  div {...}
  .foo {...}
  .bar {...} /* we don't know the value of `class` */
  :hover {...}

  /* discard these */
  div > p {...}
  div p {...}
  p {...}
  div.bar {...}
</style>

It would be helpful to print a warning if unused selectors are encountered in a component.

[Rich-Harris]

Implemented in 1.24