Resolve "Update GlCollapsibleListbox component to include appropriate roles and/or attributes"

What does this MR do?

This MR brings the Listbox component up to the intended ARIA spec by:

• Adding aria-activedescendant support - only in effect if the listbox is searchable
  • This enables focus retention on the search input when in "combobox mode"
• Adding aria-multiselectable - only in effect if multiple
• Adding aria-busy when searching is active (spec here)
• Adding a unique id for listbox items
• Adding circular navigation (spec here)

Notes:

There are a few areas where the spec's keyboard support does not make sense for this component.

• Alt + Down Arrow: Opens the listbox without moving focus or changing selection.
  • I chose to ignore this since our component pattern is different in that the listbox visibility is triggered by the dropdown button, not interaction with the input.
• Escape: Closes the listbox. Sets visual focus on the textbox.
  • It does not make sense to set visual focus on the textbox, as it will be hidden when the dropdown closes on Escape

Screenshots or screen recordings

Before	After
No circular navigation 👎	Circular navigation 👍
Current behavior: Keyboard navigation removes focus from the input.	The component now retains focus on the searchable combobox input when using arrow keys to navigate

Integration merge requests

• GitLab: gitlab!190837
• CustomersDot: mr_url
• Status Page: mr_url

Does this MR meet the acceptance criteria?

This checklist encourages the authors, reviewers, and maintainers of merge requests (MRs) to confirm changes were analyzed for conformity with the project's guidelines, security and accessibility.

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Conformity

• Code review guidelines.
• GitLab UI's contributing guidelines.
• If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• If the MR changes a component's API, integration MR(s) have been opened (see integration merge requests above).
• Added the ~"component:*" label(s) if applicable.

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• Security reports checked/validated by a reviewer from the AppSec team

Accessibility

If this MR adds or modifies a component, take a few moments to review the following:

• All actions and functionality can be done with a keyboard.
• Links, buttons, and controls have a visible focus state.
• All content is presented in text or with a text equivalent. For example, alt text for SVG, or aria-label for icons that have meaning or perform actions.
• Changes in a component’s state are announced by a screen reader. For example, changing aria-expanded="false" to aria-expanded="true" when an accordion is expanded.
• Color combinations have sufficient contrast.

Closes #3159 (closed)