Add resizing panels to Sandcastle

[jjspace]

Description

Small and simple change thanks to a really easy-to-use library allotment. I also updated the colors to pull from stratakit to match.

Issue number and link

Part of #12566

Testing plan

• Run the app however you want
• Try dragging the center vertical line sideways
  • Make sure the style looks good in light and dark and when hovered
  • Make sure both sides, editor and viewer, resize nicely
  • Make sure changing the size of the window itself still behaves nicely

Author checklist

• I have submitted a Contributor License Agreement
• I have added my name to CONTRIBUTORS.md
• I have updated CHANGES.md with a short summary of my change
• I have added or updated unit tests to ensure consistent code coverage
• I have updated the inline documentation, and included code examples where relevant
• I have performed a self-review of my code

[github-actions]

Thank you for the pull request, @jjspace!

✅ We can confirm we have a CLA on file for you.

[ggetz]

The resizing is working great, thanks @jjspace!