🏗 Add a way to run NPM scripts that live outside package.json (via nps)

[rsimha]

This PR introduces a way to run npm scripts without adding them to package.json. Scripts are defined in package-scripts.js and run using nps (short for npm-package-scripts).

This will prevent us from going back to a situation like this:

amphtml/package.json

Lines 17 to 26 in ee2dea7

	"scripts": {
	"test": "gulp test && ava",
	"ava": "ava",
	"lint": "gulp lint",
	"build": "gulp build",
	"dist": "gulp dist",
	"heroku-postbuild": "npm run test-env-var && gulp clean && gulp build --fortesting && gulp dist --fortesting && npm run prepend-amp && npm run prepend-v0",
	"test-env-var": "if [ -z \"${AMP_TESTING_HOST}\" ]; then echo \"ERROR: Please set the 'AMP_TESTING_HOST' environment variable. If you are in heroku this can be set by executing 'heroku config:set AMP_TESTING_HOST=my-heroku-subdomain.herokuapp.com' on the command line.\"; exit 1; fi",
	"prepend-amp": "gulp prepend-global --prod node_modules/AMP_CONFIG.json --local --target dist/amp.js && gulp prepend-global --prod node_modules/AMP_CONFIG.json --local --target dist.3p/current/integration.js",
	"prepend-v0": "gulp prepend-global --prod node_modules/AMP_CONFIG.json --local --target dist/v0.js && gulp prepend-global --prod node_modules/AMP_CONFIG.json --local --target dist.3p/current-min/f.js"

Usage:

# One-time install of package script runner
npm i -g nps

# Invoke any script
nps filesize

Example:

Addresses #32476 (comment)

[kristoferbaxter]

This seems like a good change, and a way for us to move off gulp in smaller... bites.