Add NuGet packaging and CI

[rmarinho]

Summary

• add NuGet packaging metadata + SourceLink for Xamarin.MacDev
• add centralized build settings in Directory.Build.props
• move shared build helpers to eng/scripts/msbuild.mk and wire Makefile
• add nuget.version and automatic version calculation from commit count
• add GitHub Actions CI (.github/workflows/ci.yml) for build/test on macOS+Windows, package on Windows
• add Azure DevOps pipeline (azure-pipelines.yaml) adapted from dotnet/android-tools
• update README to point to https://github.com/dotnet/macios
• add repository Copilot instructions and agents under .github/
• fix UnitTests project path discovery hang in TestHelper and normalize XML serialization test line-ending handling

Testing

• dotnet build Xamarin.MacDev.sln -c Release
• dotnet test UnitTests/UnitTests.csproj -c Release -f net10.0
• dotnet pack Xamarin.MacDev/Xamarin.MacDev.csproj -c Release --no-build -p:PackageVersion=1.0.0-local

[Copilot]

Pull request overview

Adds NuGet packaging metadata and a CI pipeline to build/test/pack the Xamarin.MacDev library, including shared build properties and Makefile helpers to standardize local and CI builds.

Changes:

• Add NuGet package metadata + SourceLink reference to Xamarin.MacDev.
• Introduce shared MSBuild properties (Directory.Build.props) and Make-based build abstraction (msbuild.mk + root Makefile).
• Add GitHub Actions workflow to build, test, version, and pack NuGet artifacts on macOS and Windows.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
nuget.version	Adds a base version file used by CI to compute PackageVersion.
build-tools/scripts/msbuild.mk	Provides a Make-friendly abstraction for invoking MSBuild via dotnet build.
Xamarin.MacDev/Xamarin.MacDev.csproj	Adds NuGet package metadata, SourceLink package reference, output path, and signing item setup.
Makefile	Adds local build/clean/test/pack targets.
Directory.Build.props	Adds shared repository/build output properties (and a DotNetTargetFramework property).
.github/workflows/ci.yml	Adds multi-OS CI for build/test/versioning/packing and artifact upload.

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