Improve focus states for text inputs and textareas

[adrianababakanian]

Currently, the focus states for text inputs, textareas, and selects are very subtle and difficult to identify unless you know what to look for. The borders for each are only updated to use the relevant -dark color variant when focused.

We should make these more prominent.

In https://github.com/mapbox/assembly/pull/992, we updated stroked selects to use a double box-shadow when focused. We might want to implement the same changes for text inputs and textareas.