Add support for VersionOverride attribute in Central Package Management

[Meir017]

depends on #93

Description

Add IntelliSense support for the VersionOverride attribute on PackageReference elements when using NuGet's Central Package Management (CPM) feature.

Background

Central Package Management allows managing package versions centrally in a Directory.Packages.props file. The VersionOverride attribute enables individual projects to override the centrally-defined version for specific packages:

<!-- Directory.Packages.props -->
<Project>
  <PropertyGroup>
    <ManagePackageVersionsCentrally>true</ManagePackageVersionsCentrally>
  </PropertyGroup>
  <ItemGroup>
    <PackageVersion Include="PackageA" Version="1.0.0" />
  </ItemGroup>
</Project>

<!-- Project.csproj -->
<Project Sdk="Microsoft.NET.Sdk">
  <ItemGroup>
    <!-- Override the centrally-defined version -->
    <PackageReference Include="PackageA" VersionOverride="3.0.0" />
  </ItemGroup>
</Project>

Proposed Changes

1. Update PackageReferenceCompletionProvider

   • Add VersionOverride as a supported attribute alongside Version
   • Provide package version completions for VersionOverride attribute values
   • Update attribute completion logic in HandlePackageReferenceAttributeCompletion

2. Add completion snippets

   • Consider adding a CPM-specific PackageReference snippet that includes VersionOverride
   • Update existing element completion snippets

3. Add help/documentation

   • Update help/items.json to include VersionOverride metadata for PackageReference
   • Include description explaining it's used in CPM scenarios
   • Note that it requires CentralPackageVersionOverrideEnabled (enabled by default)

4. Add tests

   • Test completion for VersionOverride attribute
   • Test package version suggestions for VersionOverride values
   • Add test fixtures with CPM enabled (Directory.Packages.props)

5. Update documentation

   • Document the new VersionOverride support in README or docs
   • Add examples showing IntelliSense for VersionOverride

References

• Microsoft Docs: Central Package Management - Overriding Package Versions
• Feature introduced in NuGet 6.2+
• Can be disabled via CentralPackageVersionOverrideEnabled=false

Acceptance Criteria

• IntelliSense suggests VersionOverride as an attribute on PackageReference elements
• Package version completions appear when typing VersionOverride="
• Both Version and VersionOverride attributes are supported
• Help text/documentation explains when to use VersionOverride
• Tests verify the completion behavior
• Documentation is updated with examples

[tintoy]

You could probably add a PackageManagementMode enum property to ProjectDocument that wraps up the corresponding MSBuild project property (that the completion provider can use to decide which completion behaviour to use). What do you reckon?