Scripts: Enhance check-engines to consider "latest" versions

Previously: #12721 Related: https://github.com/WordPress/gutenberg/pull/14192#issuecomment-468732835

Currently, wp-scripts check-engines is hard-coded to default to Node >=10.0.0 and npm >=6.0.0:

https://github.com/WordPress/gutenberg/blob/62f81e10f92a1ec2bdd71c4fe0e249c1e5cb8ccd/packages/scripts/scripts/check-engines.js#L23-L24

This doesn't faithfully represent the versions Gutenberg expects, which are respectively "latest Node LTS" and "latest npm version"

You should be running a Node version matching the current active LTS release or newer for this plugin to work correctly.

You also should have the latest release of npm installed.

https://github.com/WordPress/gutenberg/blob/master/CONTRIBUTING.md#getting-started

If the check-engines script is intended to adhere to this recommendation, it should ideally do so in a way which doesn't require manual updates.

It does not appear that the underlying check-node-version project supports this, but perhaps part of the script can be to retrieve the latest versions information.

• For npm, the data[ 'dist-tags' ].latest result from https://registry.npmjs.org/npm JSON blob
• For Node, the result of data, first version of sorted by date, filtered by lts value equal to the current LTS codename (currently "Dubnium", may need to be manually updated) from https://nodejs.org/dist/index.json

cc @gziolo @ntwb @adamsilverstein