Add Mantine theme

[zfarhad]

Reasons for making this change

Added a new theme using Mantine UI library (@mantine/core, @mantine/hooks, @mantine/dates) for react-jsonschema-form, addressing the request in issue #4289 (fixes #4289). This theme provides a modern, customizable UI component set that integrates well with RJSF's form generation capabilities.

The Mantine theme offers:

• Modern, clean UI components
• Built-in dark mode support
• Responsive design
• Accessible components
• Customizable styling through Mantine's theming system

Checklist

• I'm updating documentation
  • I've checked the rendering of the Markdown text I've added
• I'm adding or updating code
  • I've added and/or updated tests. I've run npx nx run-many --target=build --exclude=@rjsf/docs && npm run test:update to update snapshots, if needed.
  • I've updated docs if needed
  • 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

[jasny]

@zfarhad No new features will be accepted to version 5. Please make a PR for version 6 instead, which currently is in alpha.

(I'm working on the primereact theme #4485 and was asked to do the same)