feat(sdk): add template support for project and item templates

[CalvinAllen]

Parent issue: #11

Add MSBuild-based support for including VS project and item templates in VSIX packages.

This bypasses the broken VSIX Manifest Designer by providing MSBuild item types that work with SDK-style projects.

Features

• VsixProjectTemplate item type for folder-based project templates
• VsixItemTemplate item type for folder-based item templates
• VsixTemplateZip item type for pre-built template zip files
• VsixTemplateReference item type for referencing template projects
• Auto-discovery of templates in ProjectTemplates/ and ItemTemplates/ folders
• Auto-zip folder-based templates during build
• Hook into VSSDK's GetVsixSourceItems for VSIX inclusion
• Build warnings for missing manifest <Content> entries (VSIXSDK010, VSIXSDK011)

Files

• src/CodingWithCalvin.VsixSdk/Sdk/Sdk.Vsix.Templates.props (new)
• src/CodingWithCalvin.VsixSdk/Sdk/Sdk.Vsix.Templates.targets (new)
• src/CodingWithCalvin.VsixSdk/Sdk/Sdk.Vsix.props (modify)
• src/CodingWithCalvin.VsixSdk/Sdk/Sdk.Vsix.targets (modify)
• src/CodingWithCalvin.VsixSdk/CodingWithCalvin.VsixSdk.csproj (modify)