Add support for listbox filtering

What does this MR do?

This MR adds the ability to filter listbox items. The listbox is responsible for rendering the search field/ emitting the search query change and emitting selected and deselected items. It is not responsible for the filtering logic - this would be the responsibility of the consuming component.

Here is the integration MR where the 👩 👨 user select component (used in sidebar assignee widget) is migrated to the new implementation.

✏ My initial intent (and we discussed this previously) was to create the wrapper for the GlListbox - GlFilterableListbox component but it proved to be a bit problematic - besides drilling down props and slots provided by the parent it complicates the focus logic and adds an extra layer to communicate events from BaseDropdown, to GlListbox to the GlFillterbaleListbox and then up. But I'm open to suggestions).

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
• 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 in the following projects to ensure that the @gitlab/ui package can be upgraded quickly after the changes are released:
  • GitLab: mr_url
  • CustomersDot: mr_url
  • Status Page: mr_url
• 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.

References #1874 (closed)