[6.x] Modal remove behind from accessibility tree

[jaygeorge]

Description of the Problem

When the license modal is active, the page behind stays in the accessibility tree.

What this PR Does

• This is related to Licensing alert blocks the CP but has no dialog role, name, inert page, or Escape handler #15393, fixing the accessibility tree being available when the licensing dialogue shows.
• The PR removes everything behind the modal from the accessibility tree - aria-hidden hides everything except the dialog so the page behind leaves the accessibility tree.
  • Initially I explored using inert but apparently this diverges from the Reka way of doing things, and not necessarily simpler
• Keeps the page aria-hidden until the modal leave animation finishes
• Declare aria-hidden as a direct dependency. Modal imports it directly; it should not rely on reka-ui hoisting it.

How to Reproduce

1. On the broken branch run [] returned, which means that these nodes are not hidden from the accessibility tree.
2. On the fix branch run [...document.querySelectorAll('#statamic > [aria-hidden="true"]')] .map(el => el.tagName + (el.id ? '#' + el.id : '')) and you should see ['DIV', 'HEADER', 'MAIN#main'] returned, which means the DOM is hidden from the accessibility tree.