Multiple categories in the product_category shortcode

Add the ability to input multiple categories separated by a comma in the
[product_category] shortcode.
fixed some coding standards in Gruntfile.js
updated all node packages
processed all css files
added alert about changing the product type that has variations

closes #5864

@mikejolley
Hide variation stock when manage stock is unchecked

closes #5935

@mikejolley
fixed the variation manage stock visibility
improved the .editorconfig
removed a console.log()
Improve display of multiword product attributes

Instead of displaying a variation's custom product attributes like this:

`name-with-dashes: attribute-with-dashes`

Display it like this:

`Name With Dashes: Attribute With Dashes`
Add coupon description to REST API coupon response
Add shop_webhook custom post type

Part of #5564
Add webhooks REST API endpoint

Part of #5564
Introduce WC_Webhook class

Part of #5564
Hide webhook comments

Part of #5564
Add shop_webhook capabilities on install/upgrade

Part of #5564
Load webhooks on each page load

Part of #5564
Merge pull request #5938 from thenbrent/custom_attribute_label

Improve display of multiword product attributes
Send order as a whole to paypal with negative line items

Closes #5862
Merge pull request #5941 from maxrice/feature-issue-5564-rest-api-webhooks

Webhooks Implementation