feat: implement unifiedSignatures for TypeScript plugin

[JoshuaKGoldberg]

PR Checklist

• Addresses an existing open issue: fixes 🚀 Feature: Implement unifiedSignatures rule (TypeScript) #2081
• That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

Implements the unifiedSignatures rule that reports function overloads that could be unified using union types or optional parameters.

Equivalent to ESLint's @typescript-eslint/unified-signatures rule and Biome's useUnifiedTypeSignatures rule.

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Project	Deployment	Actions	Updated (UTC)
flint	Ready	Preview, Comment	Jan 31, 2026 10:11pm

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[JoshuaKGoldberg]

This implementation is looking quite large. Just to be safe, marking as blocked until we've built up more practices.