Fix keyboard navigation when using menu dropdown with multiple option

[jroofsf]

Fixes #

Fixes keyboard navigation in menu dropdown component when using the multiple option.

Additional description

Fixes this issue: Use the keyboard to select an option in the menu dropdown component. Use the keyboard's up or down arrow keys to advance to the next or previous item. The focus jumps to the first item in the list when pressing the down arrow key or to the last item when pressing the up arrow key.

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• [X] Tests have been added for new props to prevent regressions in the future. See readme.
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• [X] Review tests are passing in the browser. Open http://localhost:8001/.
• [X] Review markup conforms to SLDS by looking at DOM snapshot strings.

REVIEWER checklist (do not remove)

• [ ] CircleCI tests pass. This includes linting, Mocha, Jest, Storyshots, and components/component-docs.json tests.
• [ ] Tests have been added for new props to prevent regressions in the future. See readme.
• [ ] Review the appropriate Storybook stories. Open http://localhost:9001/.
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• [ ] Review tests are passing in the browser. Open http://localhost:8001/.
• [ ] Review markup conforms to SLDS by looking at DOM snapshot strings.

  Required only if there are markup / UX changes

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[welcome[bot]]

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[salesforce-cla[bot]]

Thanks for the contribution! Before we can merge this, we need @jroofsf to sign the Salesforce Inc. Contributor License Agreement.

[interactivellama]

Works great!

[welcome[bot]]

Congrats on merging your first pull request to Design System React! :tada: If you have a moment, please fill out this five question survey, we would appreciate it. On behalf of Salesforce's customers, partners, product specialists and employees, we would like offer sincere thanks and appreciation for helping make our user experience better. We look forward to working with you more in the future. This definitely calls for a high five!