Attempt to upgrade to node v12

[joeyAghion]

• Added an engines entry in package.json
• Removed conflicting npm install yarn ... from Dockerfile
• Had trouble installing a compatible fsevents version (fsevents @1.2.7 | @1.2.8 does not build with Node 12 nodejs/node#27389), but upgrading to >=1.2.9 helped. (Specifying a fsevents version in resolutions helped, but caused the optional dep to attempt installation in the linux container where it was incompatible.)
• Added --frozen-lockfile to surface any inconsistencies between what's declared and what's going on within the container

I saw 3 consistent failures when running tests locally, but hokusai test would sometimes succeed. I'm not familiar enough with Kaws's builds to know if this is a new problem or it's always unstable.

[eessex]

Type check is failing due to the yarn orb version - bump to v5 should fix it.

[eessex]

This looks good from here, going to merge and monitor.