Rename the equalMarkup Chai assertion to the toEqualMarkup Vitest matcher

[pomek]

🚀 Summary

Renames the equalMarkup Chai assertion to the toEqualMarkup() Vitest matcher (matching the Vitest naming style) and removes the unused attribute Chai assertion.

The matcher is registered in a Vitest setup file via expect.extend( { toEqualMarkup } ), importing it from the main entry point of @ckeditor/ckeditor5-dev-tests. To make that import safe in a browser context, the main entry point now exposes only the matcher — the test runner tasks and parseArguments() are no longer exported (the binary scripts import them directly and remain the public interface of the runners). The package ships the vitest type augmentation (lib/vitest/matchers.d.ts), so consumers get toEqualMarkup() typings out of the box.

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📌 Related issues

• Closes ckeditor/ckeditor5-internal#4595

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💡 Additional information

• The attribute assertion had zero call sites in ckeditor5 and ckeditor5-commercial, hence it is removed instead of migrated (a deviation from the issue's DoD).
• The old equalMarkup Chai factory stays in this PR — the Karma entry-file generator still registers it. It is removed together with the Karma runner in Migrate the testing environment to Vitest: rename equalMarkup to toEqualMarkup and remove the automated test runner #1413.

[pomek]

Superseded by #1413 — both changes were combined there (the matcher migration and the runner removal are tightly coupled), and the PR now targets master directly. Closing.