NumberControl: Make increment and decrement buttons keyboard accessible.

[afercia]

Fixes #57401

What?

The NumberControl increment and decrement buttons are not operable with a keyboard. This seems to be intentional.

Why?

All interactive controls presented to users should be keyboard operable.

How?

Make them keyboard accessible.

Testing Instructions

• Use Twenty Twenty Four as active theme
• Go to Site Editor > Templates > Blog Home
• Use the Tab key to move focus to the 'Posts per page' field.
• Press the Tab key again to move focus to the Increment button.
• Observe the button receives focus.
• Observe there is a clear focus style.
• Observe a tooltip appears to visually expose the button accessible name.
• Presse the Enter or Spacebar keys and check the button works as expected.
• Move focus to the Decrement button.
• Repeat the steps above.

Testing Instructions for Keyboard

Screenshots or screencast

[afercia]

I'll add a changelog entry after the tests pass.

[github-actions]

Flaky tests detected in abff455.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/7339579313
📝 Reported issues:

• [Flaky Test] should be disabled when post is being saved #35641 in /test/e2e/specs/editor/various/publish-button.spec.js

[alexstine]

Code change looks good.

[afercia]

Other instances of the custom spin controls are in the Typography settings line height and text columns (for the block that support them) and in the global styles. Screenshots: