fix(sdk): add support for NuGet Central Package Management

[CalvinAllen]

Problem

The SDK currently adds an implicit PackageReference for Microsoft.VSSDK.BuildTools with a hardcoded version:

<PackageReference Include="Microsoft.VSSDK.BuildTools" Version="$(VssdkBuildToolsVersion)" PrivateAssets="all" />

This breaks projects using NuGet Central Package Management (CPM) because CPM requires that PackageReference items do NOT have Version attributes - versions must be defined centrally in Directory.Packages.props.

Proposed Solution

Remove the implicit PackageReference from the SDK entirely. Users will explicitly add the dependency themselves:

<PackageReference Include="Microsoft.VSSDK.BuildTools" PrivateAssets="all" />

Rationale

• The SDK's role is to provide MSBuild integration (props/targets), not manage package references
• Explicit dependencies are clearer and more maintainable
• Works naturally with CPM - users define the version in Directory.Packages.props
• Works with non-CPM - users specify the version directly on the PackageReference

Changes Required

1. Remove from Sdk.Vsix.props:

   • Remove the VssdkBuildToolsVersion property (lines 21-23)
   • Remove the PackageReference for Microsoft.VSSDK.BuildTools (lines 25-27)

2. Update template (src/CodingWithCalvin.VsixSdk.Templates/templates/vsix-extension/VsixExtension.csproj):

   • Ensure template includes the PackageReference explicitly (already does)

3. Update documentation:

   • Document that users must add Microsoft.VSSDK.BuildTools package reference
   • Add migration guide for existing users

Breaking Change

This is a breaking change for existing users who rely on the implicit package reference. They will need to add the PackageReference explicitly after upgrading.