Update recommendation to use node@12

Node@12 seems to have significant performance improvements:

Node 10: Execution time in seconds Node 12: Execution time in seconds Saved time
yarn run webpack 41,37 ± 1,35 30,88 ± 3,13 25,35 %
yarn run webpack-prod 868,08 ± 48,49 506,54 ± 26,34 41,65 %

(Memory usage was about the same)

Thus it would make sense to make it the recommended version for development and use it as a default in CI.

To do that we need to:

Edited by DJ Mountney