Add required version of Node to engines in package.json

[aral]

Through trial-and-error (going back from version 19 to 15), I found that npm install only works on Node 15.

This should be documented in the readme and also added to the package.json file under engines so new developers are made aware that they likely need to switch to an earlier version of Node than what they normally use.

[aral]

Closing this as I just tested and npm does not give you an error before installing dependencies even if you have an invalid Node/npm version installed.

See: npm/cli#2175