feat: add popular UDL theme templates (Gruvbox, Nord, Dracula, One Dark Pro)

[SomshekharArabali]

Summary

Currently, markdown-plus-plus provides a few UDL theme templates, but many popular editor themes are missing.
It would be super helpful to add widely used palettes as ready-to-install UDLs.

Requested themes

• Gruvbox
• Nord
• Dracula
• One Dark Pro

Why this matters

These themes are widely adopted across editors like VS Code, Atom, and Sublime.
Including them will:

• Save users time from manual style configuration in Notepad++.
• Provide consistency for developers who already use these palettes.
• Make the project more appealing for newcomers.

Suggested approach

• Add new JSON configs into /config following the existing format.
• Generate .udl.xml files under /udl/.
• Update the README and docs to list these themes.
• (Optional) Add screenshots of each theme under /docs/images/.

Contribution

Happy to help by preparing a PR (starting with Gruvbox) if maintainers agree. 🙌

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