Enabling Link on the Express Shortcode Checkout

[wjrosa]

Fixes #3499

Changes proposed in this Pull Request:

This PR fixes the display and usage of the Link payment method when purchasing a product on the shortcode checkout with Express Checkout Element enabled.

The method was disabled by default (should be set to auto in paymentMethods), and in additional to that, it requires the paymentMethodTypes option to be provided.

Testing instructions

• Checkout and build the develop branch on your test environment
• Connect your Stripe account
• Enable Link on your Stripe dashboard and the extension settings page (wp-admin/admin.php?page=wc-settings&tab=checkout&section=stripe&panel=methods). Legacy checkout experience must be disabled
• Enable ECE. You can do it by hardcoding is_stripe_ece_enabled to true
• Create a shortcode checkout page
• As a shopper, add any product to your cart
• Go to the shortcode checkout page and confirm that Link is not available
• Checkout and build this branch instead (add/enable-link-on-the-express-shortcode-checkout)
• Repeat the purchase process
• Confirm that Link is available now and that you can complete a purchase with it

---

• [ ] Covered with tests (or have a good reason not to test in description ☝️)
• [x] Added changelog entry in both changelog.txt and readme.txt (or does not apply)
• [x] Tested on mobile (or does not apply)

Post merge

• [ ] Added testing instructions to the Release Testing Instructions wiki page (or does not apply)
• [ ] Added the needs docs label (or does not apply)
• [ ] Included this PR in the Release Thread scope (or does not apply)