Internal issue 2988 - Searchable Multi-Select

Currently the multi-select component can only be “searchable” when “multiple” is set to false. Our project would like “searchable” to be possible when “multiple” is true so users are able to search and select multiple options from a drop down.

Ref: [Multi-select - Intelligence Community Design System|https://design.sis.gov.uk/components/multi-select/code]