[cli] init: Add an option to init from the Polymer 3 compatible PSK prerelease.

[bicknellr]

Here's the Polymer 2 version:

tools/packages/cli/src/init/init.ts

Lines 62 to 71 in 67f781d

	'polymer-2-starter-kit': {
	id: 'polymer-init-polymer-2-starter-kit:app',
	description:
	'A Polymer 2.x starter application template, with navigation and "PRPL pattern" loading',
	generator: createGithubGenerator({
	owner: 'Polymer',
	repo: 'polymer-starter-kit',
	semverRange: '^3.0.0',
	}),
	},

Once there's a prerelease tagged for a Polymer 3 compatible PSK, this should mostly be a copy-paste.

[bicknellr]

Released in v1.7.0-pre.13.