Feature: Split panel keyboard navigation

[JesmoDev]

Fixes: https://github.com/umbraco/Umbraco.CMS.Backoffice/issues/1957

Adds keyboard navigation to the split panel.

• Left and right arrow keys to move the split.
• Holding shift increases the distance moved.
• Home and end moves the split to the min and max values.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Chore (minor updates related to the tooling or maintenance of the repository, does not impact compiled assets)

Checklist

• If my change requires a change to the documentation, I have updated the documentation in this pull request.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.