Decouple build process from packaging process

[schlessera]

When you run npm run build, it goes through all build:* tasks and processes them. This does not only trigger a fresh build (converting source files to build artifacts), but also triggers the packaging process (remove development dependencies and generate a ZIP archive).

As a follow-up to #3183, I'd like to decouple "building" from "packaging", so that npm run build only does the required steps to make the current source folder run as a valid plugin, and provide a separate npm run package (name up for debate) to optimize for a production environment and to generate a ZIP.

This seems to make sense, as an npm run build is something you need a lot during development, whereas you seldom need a ZIP file.

Related #3183, #1840

[thelovekesh]

QA Passed ✅

Before:
On executing npm run build:(dev|prod) following build:* scripts were get executed and results in:

• Plugin zip as amp.zip
• Plugin build as ./build dir
• JS build files

amp-wp/package.json

Lines 106 to 109 in 4e54b03

	"build:prepare": "grunt clean",
	"build:js": "wp-scripts build",
	"build:run": "grunt build",
	"build:zip": "grunt create-build-zip",

After:
Now on executing the npm run build:(dev|prod) following build:* scripts get executed and generate JS build files.

amp-wp/package.json

Lines 107 to 108 in 68d9bb3

	"build:prepare": "grunt clean",
	"build:js": "wp-scripts build",

To generate a zip or plugin build following npm scripts are being used now.

amp-wp/package.json

Lines 130 to 133 in 68d9bb3

	"package:dev": "cross-env NODE_ENV=development npm-run-all build:dev 'package:!(dev|prod)'",
	"package:prod": "cross-env NODE_ENV=production npm-run-all build:prod 'package:!(dev|prod)'",
	"package:run": "grunt build",
	"package:zip": "grunt create-build-zip",