feat: add item modal

[ash-bergs]

• Moves the form for adding an item to the list into a modal
• Modal is set up to display when the showModal function is called (by the openModalButton), and closes and resets the form when:
  • User presses esc
  • User clicks outside the modal
• Keyboard focus is trapped within the modal when it's open
• Updates accessibility throughout the form - adding aria-label and other ARIA props where appropriate and helpful

Continues on with mobile first design:

[ash-bergs]

I decided the button would look better with an icon:

Added with purse CSS 👍