feat(preview-api): expose public method for getting the qr code of playground app

[Fatme]

cons tns = require("nativescript-cli");

const result = tns.previewQrCodeService.getPlaygroundAppQrCode();

or

const result = tns.previewQrCodeService.getPlaygroundAppQrCode({ platform: 'ios' });

or

const result = tns.previewQrCodeService.getPlaygroundAppQrCode({ platform: 'android' });

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