WASP-408: Introduce vue-multiselect Package

Before making a pull request

• Put the appropriate EXEMPT flag if needed in the MR label (see https://practices.wma.chs.usgs.gov/practice/peer_code_review/)
• Run all linters (make lint)
• Run all tests (make test)
• Update the changelog appropriately
• If making a release, update code.json metadataLastUpdated and bump CHANGELOG.md to next version.

Description

This adds the vue-multiselect package that will be used for select options in Search & Filter.

After making a pull request

• If appropriate, put the link to the PR in the JIRA ticket
• Assign someone to review unless the change is trivial
• Check the accessibility CI step and fix or explain any issues.