Add support for inert attribute

[Jym77]

https://developer.chrome.com/blog/inert/
https://html.spec.whatwg.org/multipage/interaction.html#the-inert-attribute
https://developer.mozilla.org/en-US/docs/Web/API/HTMLElement/inert#browser_compatibility

[Jym77]

Seems to have 4 effects on us:

• inert elements are not included in the accessibility tree. Given that this may be overwritten by inert="false" on a descendant, this means they have to be Container rather than Inert 🙈 https://github.com/Siteimprove/alfa/blob/main/packages/alfa-aria/src/node.ts#L374
• isInert should consider inert node as… inert… This will effectively remove then from IsTabbable 👍 https://github.com/Siteimprove/alfa/blob/main/packages/alfa-style/src/element/predicate/is-inert.ts
• isSuggestedFocusable should exclude inert nodes. This will effectively remove their tabindex. https://github.com/Siteimprove/alfa/blob/main/packages/alfa-dom/src/node/element/predicate/is-suggested-focusable.ts
• isProgrammaticallyHidden should consider inert nodes as programmatically hidden. https://github.com/Siteimprove/alfa/blob/main/packages/alfa-aria/src/dom/predicate/is-programmatically-hidden.ts

We may want to wait a bit until ACT discussions happen to make sure we handle that correctly…

[Jym77]

See the HTML AAM PR to handle it.

[Jym77]

Note: modals that are opened (showModal) also make the rest of the content inert, which is going to be a bit annoying to handle.