Add keyboard accessibility to code block

[EmmaDawsonDev]

Describe your changes

Added a ref to the Pre block in the react syntax highlighter so that scrollbars can be detected. Adds a tab index when scrollbars are present in order to allow keyboard users to scroll horizontally.

Screenshots - If Any (Optional)

Link to issue

Closes #306

Checklist before requesting a review

• I have performed a self-review of my code.
• Followed the repository's Contributing Guidelines.
• I ran the app and tested it locally to verify that it works as expected.
• I have checked my code with an automatic accessibility tool such as Axe Dev Tools or Wave
  and it shows no errors.

Additional Information (Optional)

Any additional information that you want to give us.