Add shadow-initial and inset-shadow-initial utilities

[adamwathan]

This PR complements #14458 by adding new shadow-initial and inset-shadow-initial utilities that make it possible to "undo" a custom shadow color and revert to the default shadow color for the current shadow size.

For example, in this example the shadow will be red on hover even though the default color for the shadow utility is rgb(0 0 0 / 5%):

<div class="shadow-sm shadow-red-500 hover:shadow">
  <!-- … -->
</div>

This is usually what you want, but if you actually want hover:shadow to apply its default color, you need to know what the color is and add it yourself:

<div class="shadow-sm shadow-red-500 hover:shadow hover:shadow-black/5">
  <!-- … -->
</div>

Using shadow-initial, you can achieve the same thing without having to know what the original color was:

<div class="shadow-sm shadow-red-500 hover:shadow hover:shadow-initial">
  <!-- … -->
</div>

The inset-shadow-initial utility does the same thing for the inset-shadow-* utilities.