Switch GDJS browser tests to Vitest

[4ian]

Summary

• replace Karma with Vitest for GDJS tests
• update benchmarks to use vi.setTimeout
• fix beforeEach usage
• document how to run tests with Vitest
• add Vitest config and setup files

Testing

• npm test (fails: vitest not found)

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https://chatgpt.com/codex/tasks/task_b_6863f31a67348327b10a19312f4d6caa