Upgrade to node v12

[julienw]

Node v12 is the new LTS, we should upgrade. Because this is used only for the dev environment I think this will be straightforward.

Some related things to change are:

• remove the NODE_OPTIONS=--experimental-worker things because it's not experimental anymore
• TextDecoder and TextEncoder are now globals so we can remove the code in some of our tests that makes them global
• change 10.15 to 12 in .circleci/config.yml. This is also where the value is taken in bin/pre-install.js.
• While we're here, I was thinking we can at least try to switch to the more lightweight circleci images cimg/node:12 instead of circleci/node:12, see if this works. This will require change the code in bin/pre-install.js to extract the node version.
• change it also in appveyor.yml

Any taker?

[duncanjbain]

Hi @julienw, I’ll take this up! Will get a PR in soon.