Add watch support for stylesheets in packages

[talldan]

Description

closes #8343

Scss files in packages are not rebuilt while running npm run dev. This PR add watch support for scss files in packages.

How has this been tested?

• Tested editing both scss and js files in the editor package while npm run dev was active and observed that the changes were then visible in the browser after reloading the page.
• Tested editing non-package files while npm run dev was active and observed no adverse side-effects
• Tested npm run build and observed no adverse side-effects

Types of changes

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.

[talldan]

Not sure if we'd need some kind of rule excluding the components package scss from being watched, since that will end up being watched by both webpack and this script.

[youknowriad]

I think it's fine :) it will generate a new CSS for the package but not use it in the webpack build.

[youknowriad]

In another PR #8287 I have the use-case for multiple SASS entry points (multiple CSS outputs) per package. Do you have some ideas on how this would play together?

[talldan]

The simplest way I can think of is to add some kind of config, potentially as a custom property to the package.json that could be picked up by the build scripts:

{
  scssEntryPoints: [
    'style.scss',
    'some/other/style.scss'
  ],
}

Either that or some kind of naming convention e.g. styles.main.scss.

I think the config idea is safer.

[youknowriad]

LGTM 👍