Update accessibility label on cart toggle

[spencercanner]

• Add new text option to cart toggle to be used as an accessibility label for the number of items
• Add getters to the toggle view class to return accessibility labels and visually hidden summary HTML
• Add new CSS styles to the host.css file, which are the styles that are added to the top-level page. This was required because the visually hidden text is embedded on the page, and not in an iframe.
  • This also fixes a bug where the readableLabel was not being hidden because the shopify-buy--visually-hidden styles were not available
• Add visuallyhidden text to the toggle count template, so the accessibility text is available in non-iframe mode

To 🎩 :

• Create a buy button with an iframe toggle that has count set to false in the toggle contents options
• Navigate a virtual cursor to the cart toggle
  • Verify that it announces Cart button
  • Verify that it does not announce the iframe or its contents
• Create a buy button with an iframe toggle that has count set to true in the toggle contents options
• Navigate a virtual cursor to the cart toggle
  • Verify that it announces Cart and the number of items in the cart
  • Verify that it does not announce the iframe or its contents
  • Update the cart quantity and verify that the DOM only contains a single node with the accessibility label outside the iframe
• Create a buy button with a non-iframe toggle that has count and title set to false in the toggle contents options
• Navigate a virtual cursor to the cart toggle
  • Verify that the visually hidden title is announced
• Create a buy button with a non-iframe toggle that has count set to true in the toggle contents options
  • Verify that the visually hidden text is shown (it will be displayed because it's using the styles are not applied in non-iframe mode)
• Create a buy button with a non-iframe toggle that has the title set to true in the toggle options
  • Verify that the title is only displayed once

• Chrome - Mac (sanity check)
• Safari - Mac - VoiceOver
• Firefox - Windows - NVDA