Add polyfills for some of the OperatingSystem.IsXyz() methods

[Tyrrrz]

No description provided.

[Copilot]

Pull Request Overview

This PR adds polyfills for operating system detection methods from .NET 5.0, making them available to earlier framework versions. The implementation provides backward compatibility for OS platform checks and version detection.

• Adds OperatingSystem.IsFreeBSD(), IsLinux(), IsMacOS(), IsWindows(), and IsWindowsVersionAtLeast() methods
• Introduces OSPlatform.FreeBSD property for .NET Core 3.0 and earlier
• Includes a placeholder OperatingSystem class for .NET Core 2.0 and earlier frameworks

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
PolyShim/Net50/OperatingSystem.cs	Implements polyfills for OperatingSystem.IsXyz() methods and IsWindowsVersionAtLeast() for pre-.NET 5.0 frameworks
PolyShim/NetCore30/OSPlatform.cs	Adds OSPlatform.FreeBSD property polyfill for .NET Core 3.0 and earlier
PolyShim/NetCore20/OperatingSystem.cs	Provides placeholder OperatingSystem class for .NET Core 2.0 and earlier to support extension methods
PolyShim/NetCore20/SystemException.cs	Adds blank line for code formatting consistency
PolyShim.Tests/Net50/OperatingSystemTests.cs	Adds tests for IsLinux(), IsMacOS(), and IsWindows() methods across different platforms

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[Copilot]

Pull Request Overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 5 comments.

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[Copilot]

Pull Request Overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (d137db0) to head (ae9085a).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

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##   master   #34   +/-   ##
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