Wildcard: `<Combobox />` Implementation

Created by: umpox

Wildcard implementation of the <Combobox /> component.

See Wildcard V2 - Planned work for more context

Implementation

1. Merge the initial implementation by @vovakulikov: https://github.com/sourcegraph/sourcegraph/pull/25790
2. Fix outstanding UI issues: https://github.com/sourcegraph/sourcegraph/pull/25790#issuecomment-946474313
3. Audit and fix any outstanding accessibility issues

Acceptance criteria

• Component is implemented within Wildcard
• Component has tests
• Component is accessible
• Component matches Figma designs