Store nil JSON assignments as null

[atdrendel]

• Add removeValue(forKey:) for explicit dictionary key removal
• Distinguish missing optional JSON values from explicit NSNull comparisons
• Document and test nil subscript assignment behavior

[Copilot]

Pull request overview

This PR adjusts JSON dictionary subscript semantics so assigning nil stores an explicit JSON null value (instead of removing the key), adds an explicit removeValue(forKey:) API for key removal, and updates tests/docs to cover the new behavior and null-vs-missing distinction.

Changes:

• Store nil dictionary subscript assignments as JSON.null across JSON and JSON? subscripts.
• Add removeValue(forKey:) to explicitly remove keys from JSON dictionaries.
• Add/extend tests and README guidance for nil assignment behavior and distinguishing missing keys from explicit null (including NSNull comparisons).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
Tests/JSONTests/JSONTests.swift	Adds coverage for nil subscript assignment storing JSON.null, removeValue(forKey:), and NSNull comparison behavior.
Sources/JSON/JSON.swift	Implements nil-as-null subscript setters, adds removeValue(forKey:), and updates NSNull equality to distinguish missing vs explicit null.
README.md	Documents the new nil subscript assignment semantics and the new explicit key removal API.

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