feat: implement typeConstituentDuplicates for TypeScript plugin

[JoshuaKGoldberg]

PR Checklist

• Addresses an existing open issue: fixes 🚀 Feature: Implement typeConstituentDuplicates rule (TypeScript) #2076
• That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

Implements the typeConstituentDuplicates rule that reports duplicate types in union and intersection types. Duplicates are redundant and should be removed.

Equivalent to ESLint's @typescript-eslint/no-duplicate-type-constituents rule.

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Project	Deployment	Review	Updated (UTC)
flint	Ready	Preview, Comment	Jan 26, 2026 6:55am

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[JoshuaKGoldberg]

Hmm, this should use assignability API strategies not yet fully done in typescript-eslint. Marking as blocked until we have more practices built up.