NuGet: Source Link + XML docs

[benaadams]

Problem

NuGet packages should include Source Link for debugging and XML documentation for IntelliSense.

Sub-tasks

• Enable Source Link (<EmbedUntrackedSources>, Microsoft.SourceLink.GitHub)
• Include XML documentation in packages (<GenerateDocumentationFile>true</GenerateDocumentationFile>)
• Deterministic builds (<Deterministic>true</Deterministic>, <ContinuousIntegrationBuild>)
• Snupkg symbol packages

Dependencies

Depends on: #137, #136
Unlocks: #139 (NuGet publish on tag)

Phase

Phase 4 (Polish & Release Readiness)