Merge projects and some tidying

[drewnoakes]

Previously we had three projects, two of which were very small. This PR merges them all into the VSIX project.

Additional tidy ups:

• Add documentation
• Make several methods non-async and remove RunAsync calls, to improve performance
• Use ImmutableArray<> over ImmutableList<> to reduce memory, as we were never updating the collection
• Introduce a base class for build loggers (for use in a future PR)
• Update namespaces to better match project structure
• Make some types internal
• Update .editorconfig to amend some csharp_style_* rules
• Bump VS package versions
• Make some collection updates thread-safe and lock-free

[drewnoakes]

/azp run

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