Speed up the build a bit

[jonathonherbert]

What does this change?

A few things to shorten our build times, which are routinely ~10 minutes at present:

• Test and scalafmt within a single SBT session, rather than multiple sessions.
• Caching for npm and sbt.

How to test

Check the actions tab. This branch should run a bit quicker:

[jonathonherbert]

Exactly, @samanthagottlieb 👍