Checkbox gets keyboard accessibility

[petemill]

Changes

• Adds testing-library dependencies and setup
• Adds focus ability and style to checkbox (also hover to bring closer to design system)
• Tests focusability

Link / storybook path to visual changes

• https://brave-ui-ses0ykvow-petemill.vercel.app/?path=/story/components-form-controls--checkbox

Integration

• Does this contain changes to src/components or src/

  • Will you publish to npm immediately after this PR, or wait until sometime in the future?
  • Incompatible API change to something existing (major version increase)
  • Adding new backwards-compatible functionality? (minor version increase)
  • Fixing a bug backwards-compatibly? (patch version increase)

• Does this contain changes to src/features for brave-core?

  • Are there non backwards-compatible changes required for brave-core? Do not merge until brave-core PR is approvable. Link to brave-core PR:
  • Will you create brave-core PR to update to this commit after it is merged?
  • Wants uplift to brave-core feature branch?
    • When uplift-approved, merge to brave-core-0.VV.x feature branch
    • Create additional brave-core PRs for each feature branch to update commit

[nullhook]

lgtm

[petemill]

CI failure is existing npm audit issue