Add node and npm versions to package.json and .node-version file

[PaulREnglish]

What problem does this address?

Currently, it is unclear which versions of npm and node the developer should use should they wish to contribute.

What is your proposed solution?

My solution has 2 parts:

1. Add the version of npm and node to the "engines" property in package.json. Use the build tools as an example.
2. Add a .node-version file so certain node version managers can automatically use the correct version of node.

[g-elwell]

Please can we use a .node-version file if implementing this instead of .nvmrc

[PaulREnglish]

Please can we use a .node-version file if implementing this instead of .nvmrc

Amended the OP.

[PaulREnglish]

PR: #116