feat(GlCollapsibleListbox): Expose user interactions on list items

What does this MR do?

Adds an event - item-toggle - that exposes user interactions on list items.

This is roughly equivalent to the native click event that fires when a checkbox is toggled with a mouse click or a spacebar keypress.

Unlike the native click, item-toggle doesn't really have a great way to simply pass out the element being toggled, so it provides a payload:

{
    item: [The value of the list item being interacted with],
    selected: [The new selection state - i.e. "checked" - as a boolean]
}

Does this MR meet the acceptance criteria?

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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.
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• 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.

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.