Support multiple frameworks

[walterlv]

No description provided.

[Copilot]

Pull Request Overview

This PR adds support for multiple .NET frameworks by expanding target frameworks from net8.0 and net6.0 to include net5.0, netcoreapp3.1, and netstandard2.0. This enables the library to run on a broader range of .NET versions while maintaining compatibility with newer features through conditional compilation.

Key changes:

• Added conditional compilation directives throughout the codebase to handle API differences across framework versions
• Updated test assertions from Assert.ThrowsException to Assert.ThrowsExactly for improved test precision
• Introduced centralized package management using Directory.Packages.props

Reviewed Changes

Copilot reviewed 27 out of 27 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/DotNetCampus.CommandLine/DotNetCampus.CommandLine.csproj	Added net5.0, netcoreapp3.1, and netstandard2.0 target frameworks with conditional AOT compatibility
Directory.Packages.props	New centralized package version management file
Directory.Build.props	Enabled central package management and added NU1507 warning suppression
Multiple test files	Updated assertion methods from ThrowsException to ThrowsExactly
Multiple source files	Added conditional compilation for framework-specific APIs

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