feat: add built-in Arabic localization

[ezAldinWaez]

Description

This PR adds the built-in Arabic (ar) localization file following the BCP 47 IETF language tag standard.

Changes:

1. Created src/resources/language/_language-ar.yml duplicating all keys from the _language.yml default.
2. Verified exact key count parity (111 keys) with the English fallback to ensure flawless UI operations.
3. Used standard formal Arabic Technical terms (MSA) for both scientific/manuscript contexts (e.g., references, appendices, footnotes) and programming/UI components (e.g., source code, listings, callouts etc.).
4. Added _language-ar.yml directly to the supported language table in src/resources/language/README.md.

Testing

• Locally rendered a .qmd sample file with lang: ar to ensure the correct application of key translations across callouts, categories, and cross-references.

Checklist

• filed a contributor agreement. (📝 Note: I am ready to sign, but the current PDF links are broken as reported in Broken Contributor Agreement PDF links (404) across repository onboarding files #14388. I have also reached out to jj@rstudio.com).
• referenced the GitHub issue this PR closes (N/A)
• updated the appropriate changelog in the PR (N/A)
• ensured the present test suite passes
• added new tests
• created a separate documentation PR in Quarto's website repo and linked it to this PR

[Copilot]

Pull request overview

Adds built-in Arabic (ar) localization resources to Quarto’s bundled language pack, and documents Arabic as a supported built-in translation.

Changes:

• Added src/resources/language/_language-ar.yml with Arabic translations for the default language keys.
• Updated src/resources/language/README.md to include Arabic in the built-in translations table.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
src/resources/language/_language-ar.yml	Introduces Arabic translations for Quarto UI/crossref strings.
src/resources/language/README.md	Documents Arabic as a built-in localization option.

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