Add shadow-xs, increase alpha of shadow-sm to 0.05

[adamwathan]

This PR adds a new shadow-xs utility that adds a very subtle 1px solid shadow with no vertical offset. It can be very useful when applying another shadow to an element that has the same background color as the canvas beneath it and you feel like you need a border to add extra separation, but a border is sort of the wrong solution because it's a solid color and really you just want to increase the distinction between two elements by sharpening the shadow.

Here's an example of where it's useful...

No border/distinction shadow:

Border, notice how weird it looks at the edge that crosses into the dark background:

Using shadow-xs to add a tiny bit of distinction without introduce a true solid border:

Since shadows aren't easily composable in CSS, the code I've used for that example involves two nested divs, each with their own shadow so they stack:

<div class="rounded-md shadow-lg ...">
  <div class="rounded-md shadow-xs ...">
    <!-- Card contents -->
  </div>
</div>

The reason I don't think it makes sense to just add this little 1px distinction border directly to shadow-lg and other large shadows is that it's not necessary when the card has a different color from the background already.