Add platform requirements

[mindplay-dk]

What do you think about adding the platform requirements to package.json?

Per the contribution guide, we need:

"engines": {
  "node": ">=15",
  "npm": ">=7"
},

And a .npmrc with:

engine-strict=true

But would have saved me some time - I hurriedly tried to just get the test-suite running on two different systems, and it failed in various unexplainable ways, anything from failed tests to the same console command repeating endlessly when I ran npm run build.

Note that this doesn't enforce anything on package users - it's just for development.

Supposedly works with nvm as well, for those who have it.

Probably can't hurt?