fix(dropdowns): compose onKeyDown handler for multiselect

[hzhu]

Description

Since #1176, some of Garden's dropdown behaviors have changed slightly. This pull request addresses a small difference in behavior for the Multiselect component which results in a bug for some consumers.

Detail

When consumers pass in an onKeyDown handler, it overrides Garden's internal onKeyDown code causing problems searching the multiselect. One example is that the Space key is not registered. Try searching for an item with a space in this example.

Although, consumers should use onInputChange as shown on the API docs, the Multiselect component should still register the key strokes as expected if they choose to add an onKeyDown handler.

This PR addresses the issue by composing the onKeyDown handlers if it is passed into the component.

Checklist

• 🌐 demo is up-to-date (yarn start)
• ⬅️ renders as expected with reversed (RTL) direction
• 🤘 renders as expected with Bedrock CSS (?bedrock)
• ♿ analyzed via axe and evaluated using VoiceOver
• 💂‍♂️ includes new unit tests
• 📝 tested in Chrome, Firefox, Safari, Edge, and IE11

[coveralls]

Coverage remained the same at 95.765% when pulling ab5b39b on hzhu/multiselect-fix into 63d4d12 on main.