Number/value picker component

[doubleedesign]

Summary

What is the nature of this pull request?

• New feature
• Enhancement to an existing feature
• Bug fix
• Configuration update
• Dependency update

Description of change

This PR adds a drop-down component using React Select which adopts the current theme and accepts an onChange function that will provide the selected value back to the calling component for use.

I have manually tested the component using both mouse and keyboard interaction, and adjusted styling accordingly. I have also included extensive unit tests which cover this too.

Planner card link

Redback UI - number/value picker component

Readiness

• The application runs successfully with my change.
• I have included code comments where appropriate.
• Where I have copied and pasted code from other sources, I have included a comment with a link to the original source.
• I have read and followed the principles and guidelines about submitting code.
• I have added unit tests for new components and/or functionality.
• All existing and new unit tests pass (including updating tests for any necessary breaking changes).