Fix product tile button keyboard accessibility

[spencercanner]

• Add support for clicking the product tile button using the space key
• Change the event listener on the product tile button to keydown so it is not triggered after closing the modal using the keyboard
  • This also makes it consistent with other keyboard event handlers
• Linting prefers an early return, so the keydown event handler was refactored

To 🎩 :

• Create a buy button where isButton is set to true for the product component, and the button destination is modal
• Tab to the product tile button
• Verify that button can be clicked using the enter key
• Close the modal by pressing the enter key when focused on the close button, and verify that the modal does not re-open
• Verify that the product tile button can be opened using the space key
• Close the modal by pressing the space key when focused on the close button, and verify that the modal does not re-open
• Sanity check that add to cart, and checkout button destinations still function as expected when isButton is set to true

Browsers:

• Safari - Mac
• Chrome - Mac
• Firefox - Mac
  • Firefox has a bug where preventDefault does not work on space keydown events. This causes the cart/modal to close right after it is opened using the space key.
  • Firefox also prevents popups from being initiated through keyboard clicks, so pressing space or enter on the checkout product tile button won't work
• Edge - Mac
• Legacy Edge - Windows
• Edge - Windows
• Chrome - Windows
• Firefox - Windows
  • Same limitations as Mac Firefox