Create block: Add Node and npm version check

[gziolo]

Related comment #19773 (comment):

/home/build/.npm/_npx/13386/lib/node_modules/@wordpress/create-block/lib/scaffold.js:6
const { readFile, writeFile } = require( 'fs' ).promises;
        ^

TypeError: Cannot destructure property `readFile` of 'undefined' or 'null'.

with npm init @wordpress/block on node v8.14.0 (LTS),
npm 6.13.7 and WSL 1 Ubuntu (stable) (Bash on Windows).

Node 10.0.0+ is required, see:
https://github.com/WordPress/gutenberg/tree/master/packages/create-block#quick-start

(requires node version 10.0.0 or above, and npm version 6.1.0 or above)

We should add version checking in the script. Promises API for fs was added in Node 10.x.

Node 8.x is no longer maintained. See https://github.com/nodejs/Release#release-schedule.

The same applies to older versions of npm. For npm init @wordpress/block we even should increase the required version of npm to 6.9.0 because @wordpress/scripts requires it and it won't install properly for ESNext template (reported by @artpi in another place).