feat(navigation) - add inert attribute to NavExpandable

[Mash707]

Closes #11732

[patternfly-build]

Preview: https://patternfly-react-pr-11749.surge.sh

A11y report: https://patternfly-react-pr-11749-a11y.surge.sh

[Mash707]

@thatblindgeye should I also include a cypress test? There was some doubt regarding it in the issue.

[thatblindgeye]

One file comment below. As for your question, we could probably handle testing in the unit tests. Ideally off the top of my head we'd want the following tests:

• check that inert is on the <section> element by default
• check that inert is NOT on the <section> when isExpanded is true

Then utilizing userEvents:

• click the expandable toggle to expand it, then tab() and assert that first item is focused
• click the expandable toggle to collapse it, then tab() and assert that the first item within that section doesn't have focus

We'd also want these to be in a new NavExpandable.test.tsx file. If you have any questions or run into any issues just let me know

[Mash707]

Thanks for the amazing instructions, will start working on the unit test.

[Mash707]

• check that inert is on the
  element by default
• check that inert is NOT on the
  when isExpanded is true

@thatblindgeye I have added tests regarding these two. Let me know what you think