Navigation: Add ariaLabel block support

[carolinan]

What?

This PR enables the ariaLabel block support for the navigation block. This allows developers to manually add an aria-label to the <nav> element when they are using placeholder links and not an existing menu.

Why?

In Twenty Twenty-Four, the footer design has a 3 column menu, each column uses its own navigation block.
Each navigation block has placeholder links in the markup. To improve the accessibility, the three <nav> elements need to be differentiated and identifiable: they need an aria-label.

See WordPress/twentytwentyfour#345

How?

Enables the ariaLabel block support in block.json.
Adds a condition that populates a variable in the index.php file with the ariaLabel attribute, if it is set.

This allows developers to include the aria-label in the following format:

"ariaLabel":"Custom label here"

<!-- wp:navigation {"overlayMenu":"never","layout":{"type":"flex","orientation":"vertical"},"style":{"typography":{"fontStyle":"normal","fontWeight":"400"}},"fontSize":"small","ariaLabel":"Custom label here"} -->

<!-- wp:navigation-link {"label":"Team","url":"#"} /-->
<!-- wp:navigation-link {"label":"History","url":"#"} /-->
<!-- wp:navigation-link {"label":"Careers","url":"#"} /-->

<!-- /wp:navigation -->

And when the user updates the navigation block to use a menu the correct aria-label, the menu name, will be used because it will override the manually entered value.

Testing Instructions

Please add the example code above to your theme. Either to a block theme template,
or copy and paste the code into the code editor in a post or page.
View the navigation on the front of the website, view the HTML source, and confirm if the label is printed correctly.

Next, please test that the aria-label still works correctly for a regular navigation block with a menu assigned to it:
Add a new navigation block, create a menu, note the name of the menu in the Advanced panel, save, and view the navigation on the front. View the HTML source, and confirm if the label is printed correctly.

[alexstine]

@carolinan This works great up until you add a standard navigation block. Once that happens, the custom aria-label goes away. Not sure why that is happening.

[alexstine]

Sorry, I somehow managed to mess up the block code in testing. This works with a custom ariaLabel block attribute and it also works along side a navigation block with a menu pulled from Core.

Tagging @getdave so he is aware.

I think this looks good.

[aristath]

This works as expected and the code is simple.
I just added a minor note, but it's not a blocker - just personal preference for code readability.

[alexstine]

@aristath Nice suggestion. Didn't know ?? was supported in PHP.

[aristath]

@alexstine yeah, it was added in PHP 7, and since that is now the lowest version of PHP that WP supports, we can safely use it in the project 👍