C#: add --use-records option for C# 9+ record types

[naughtyGitCat]

Summary

• Adds a C# --use-records renderer option that emits partial record instead of partial class for data types (C# 9+).
• Works with both Newtonsoft and System.Text.Json; FromJson partials match the type kind so they merge correctly.
• Helper types (Serialize, Converter) and synthetic top-level wrappers stay classes; unions stay structs.
• Adds unit coverage and end-to-end fixture quick-test options for both frameworks.

Closes #1778.

Test plan

• npm run build
• npm run lint
• npm run test:unit -- test/unit/csharp-records.test.ts test/unit/just-types-option.test.ts
• QUICKTEST=true FIXTURE=csharp npm run test:fixtures (includes use-records: true combinations)
• QUICKTEST=true FIXTURE=csharp-SystemTextJson npm run test:fixtures (includes use-records: true combinations)
• Manual CLI smoke test: quicktype sample.json -l csharp --use-records and --just-types

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[github-actions]

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176482 changed lines — +176482 / −0

Open the generated-output report →

[schani]

Thank you so much!