feat(assets): add ProjectReference integration when adding project-based assets

[CalvinAllen]

Summary

• Add ProjectReference to VSIX project when adding project-based assets or dependencies
• Use type-specific output groups (PkgdefProjectOutputGroup for VsPackage/ToolboxControl, TemplateProjectOutputGroup for templates)
• Add ReferenceOutputAssembly=false for template assets
• Generate correct project tokens with output groups (|ProjectName;OutputGroup|)
• Add %CurrentProject% source option for self-referencing assets
• Handle template RequiredFolder pattern (Path = folder, TargetPath = token)
• Remove ProjectReference when removing project-based assets
• Add file to project with IncludeInVSIX when adding file-based assets
• Clear dialog fields when source type changes

Test plan

• Add a project asset (VsPackage type) and verify ProjectReference is added with correct output groups
• Add a template asset and verify ReferenceOutputAssembly=false is set
• Use "This Project" source and verify %CurrentProject% token is used
• Remove a project asset and verify ProjectReference is removed
• Change source type in dialogs and verify fields are cleared

Closes #8