Build out a Layer component as a replacement for the light prop

[joshblack]

This work builds on top of the contextual tokens work that's been implemented for @carbon/styles. Layer will be a simple component that will be a replacement for the light prop. the way it works is that it will be a simple container that will render the cds--layer class name. This class name remaps any contextual tokens that are in play to the next step in their layer set.

// Before
<Dropdown light />

// After
<Layer>
  <Dropdown />
</Layer>

The Layer component can be configured to be any element type that the user wants.

This work should include usage documentation.