Add polyfill for Dictionary<TKey, TValue>.TryAdd(...)

[Tyrrrz]

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[Copilot]

Pull request overview

This PR adds a polyfill for the Dictionary<TKey, TValue>.TryAdd(TKey key, TValue value) method, which was introduced in .NET Core 2.0. The polyfill enables this functionality for earlier framework versions including .NET Core 1.x, all versions of .NET Framework, and .NET Standard versions prior to 2.1.

• Implements TryAdd using the check-then-add pattern via ContainsKey and Add
• Updates the signature documentation and count metadata
• Enhances the PowerShell signature extraction script to handle generic constraints
• Provides comprehensive test coverage for both success and failure scenarios

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
PolyShim/NetCore20/Dictionary.cs	Adds the TryAdd extension method polyfill with proper generic constraints matching the native API
PolyShim/Signatures.md	Documents the new Dictionary.TryAdd member and updates total signature counts
PolyShim/List-Signatures.ps1	Updates regex pattern to correctly parse extension methods with generic where clauses
PolyShim.Tests/NetCore20/DictionaryTests.cs	Adds test coverage for TryAdd with scenarios for both new keys and duplicate keys

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