Introduce npm scripts

[nickcernis]

We closed #40 because we're moving away from Grunt as a build tool.

We should still consider adding npm scripts or similar automation to:

• Lint CSS/JS.
• Check tested to and WooCommerce supported versions.
• Make the language file.
• Package the zip.

cc @marksabbath for info, just in case I missed a branch where we're adding this already.