Fix #4080 by moving `base64` encoder/decoder from `@rjsf/utils` to playground

[nickgros]

• Move base64 encode/decoder object to playground, remove from utils
• Add test script and config to playground

Reasons for making this change

Fixes #4080

Checklist

• [ ] I'm updating documentation
  • [ ] I've checked the rendering of the Markdown text I've added
• [X] I'm adding or updating code
  • [X] I've added and/or updated tests. I've run npm run test:update to update snapshots, if needed.
  • [ ] I've updated docs if needed
  • [X] I've updated the changelog with a description of the PR
• [ ] I'm adding a new feature
  • [ ] I've updated the playground with an example use of the feature