chore: add Revit 2027 support and GitHub Packages NuGet source

[Copilot]

Rhythm lacked build configurations and CI pipeline support for Revit 2027, though deploy/2027/ pre-built DLLs and the dynamo_to_revit_mapping.json entry ("4.0": [27]) were already in place.

Project configurations (RhythmCore, RhythmRevit, RhythmUI)

• Added Debug R27 / Release R27 property groups to all three .csproj files
  • Dynamo version: 4.0, target framework: net8.0-windows, define constants include R27 and R27_OR_GREATER
• Updated all conditional ItemGroup blocks to include/exclude R27 appropriately
• Added RhythmRevit 2027 hint path reference in RhythmUI.csproj
• Fixed pre-existing inconsistency: RhythmCore.csproj R26 was using net7.0-windows; updated to net8.0-windows to match RhythmRevit and RhythmUI

Solution files

• Added Debug R27|Any CPU and Release R27|Any CPU to SolutionConfigurationPlatforms and ProjectConfigurationPlatforms in all three .sln files

CI workflow (build.yml)

• Added missing Build RhythmCore - Release R26 step (was never building R26 core)
• Added Build RhythmCore - Release R27 and Build RhythmRevit - Release R27 steps
• Extended verification arrays to include 2026 and 2027 for both RhythmCore and RhythmRevit checks
• Added an "Authenticate GitHub Packages NuGet source" step using GITHUB_TOKEN so MSBuild can restore DynamoVisualProgramming.Revit from the custom feed

NuGet source (NuGet.Config)

• Added NuGet.Config at the repository root registering https://nuget.pkg.github.com/johnpierson/index.json as a package source alongside nuget.org, so that DynamoVisualProgramming.Revit is resolved from the custom GitHub Packages feed

[johnpierson]

@copilot - can we revise all DynamoRevit dll references to use this nuget package source? https://github.com/johnpierson/DynamoVisualProgramming.Revit/pkgs/nuget/DynamoVisualProgramming.Revit

[Copilot]

@copilot - can we revise all DynamoRevit dll references to use this nuget package source? https://github.com/johnpierson/DynamoVisualProgramming.Revit/pkgs/nuget/DynamoVisualProgramming.Revit

Done in commit 2081c3b.

Added a NuGet.Config at the repository root that registers https://nuget.pkg.github.com/johnpierson/index.json as a NuGet source (alongside nuget.org). Added an "Authenticate GitHub Packages NuGet source" step in build.yml that calls dotnet nuget update source with GITHUB_TOKEN so MSBuild can authenticate and restore DynamoVisualProgramming.Revit from that feed during all build steps.