Define Node.js and npm version requirements

[kasparsd]

Match the Gutenberg setup:

https://github.com/WordPress/gutenberg/blob/6fc88cade742036261bfb31140ed0bc134e9407d/.nvmrc

We essentially need node v10+ and npm 6.9+ since that's required by the wp-scripts package:

https://github.com/WordPress/gutenberg/blob/6fc88cade742036261bfb31140ed0bc134e9407d/packages/scripts/package.json#L22-L25

[derekherman]

@kasparsd shouldn't the lock file be updated in this PR?

[kasparsd]

shouldn't the lock file be updated in this PR?

@derekherman Yeah, I also checked this twice after removing the package and apparently the same package is used by the @wordpress/scripts package so nothing changes for the lock file.