Enforce min width on `MultiSelect`

Created by: courier-new

Part 2, closes https://github.com/sourcegraph/sourcegraph/issues/37116.

We're the only consumers of this component at the moment so this feels like a reasonable change at the Wildcard component level.

Test plan

Manually verified, tested at different screen widths. Component also has Storybook coverage.

App preview:

• Web
• Storybook

Check out the client app preview documentation to learn more.