[WiX 5] Migrate SDK from v3 to v5

[joeloff]

Overview

This PR migrates the .NET SDK from WiX 3.14 to using 5.0.2. The package used is different from the public toolset and built from source.

Changes

• Add separate SDK style projects for each installer: CLI, templates, and Visual Studio placeholder and standalone bundle.
• Add .wixproj files as ProjectReferences to dotnet-sdk.proj and assign an Idle target. This allows the projects to participate in restore operations, but avoids being built as an actual reference. The projects depend on various values that are unavailable when project references are resolved.
• Convert .wxl files to the v4 schema
• Delete all the generate*.ps1 scripts that previously invoked Candle and Light. These tools no longer exist in v5.
• Generate a PackageGroup for MSIs to chain into the SDK bundle. This avoids repeating the same authoring and simplifies adding additional MSI packages in the future (assuming they follow the same pattern).
• Bundles will now match the target architecture. In v3, bundles were always 32-bit applications. This should be documented as a potential breaking change since detection logic will need to search under both registry views.