chore: add CODEOWNERS file

[3w36zj6]

This pull request adds a CODEOWNERS file to specify file ownership.

https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-code-owners

[Copilot]

Pull Request Overview

This PR adds a CODEOWNERS file to establish file ownership for code review assignments, with global ownership defaulting to @3w36zj6 and Japanese translation files specifically assigned to @typst-community/jp.

• Establishes global code ownership pattern
• Sets specific ownership for Japanese translation files

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