Add rule for CKSource Holding copyright

[Kaushik-Kumar-CEG]

Fixes #4583

Problem

CKEditor 5 files are incorrectly detected with extra licenses they don't contain.

Current (incorrect):

(gpl-2.0-plus AND (gpl-2.0-plus OR lgpl-2.1-plus OR mpl-1.1)) OR commercial-license

Expected (correct):

gpl-2.0-plus OR commercial-license

Root Cause

The generic CKSource rule matches both CKEditor 4 and CKEditor 5, but CKEditor 5 uses a different copyright holder ("CKSource Holding sp. z o.o.") and doesn't include LGPL/MPL licenses.

Solution

Added a specific rule that matches the "CKSource Holding sp. z o.o." copyright header used exclusively in CKEditor 5 packages. This rule takes precedence and returns the correct license expression.

Testing

• ✅ Verified on @ckeditor/ckeditor5-special-characters@43.0.0
• ✅ Works across different copyright years (future-proof)
• ✅ All license detection tests passing

Changes

• Added gpl-2.0-plus_or_commercial-license_cksource_1.RULE

---

Tasks:

• Reviewed contribution guidelines
• PR is descriptively titled 📑 and links the original issue above 🔗
• Tests pass -- look for a green checkbox ✔️ a few minutes after opening your PR -- Run tests locally to check for errors
• Commits are in uniquely-named feature branch and has no merge conflicts 📁
• Updated documentation pages (if applicable)
• Updated CHANGELOG.rst (if applicable)