Ship the MSBuild task-authoring analyzers

The MT task analyzer (which validates that multi-threaded tasks follow correct patterns like using `AbsolutePath` instead of `Path.GetFullPath`, avoiding static state, etc.) currently exists in the MSBuild repo but is not published as a consumable NuGet package. External teams (e.g., SDK) cannot easily use it to validate their task migrations.

## Steps

- [x] Publish to the internal feed as a NuGet package for early adopters [size XS]
- [x] [#14990](https://github.com/dotnet/msbuild/issues/14990) [#14991](https://github.com/dotnet/msbuild/issues/14991) [#14971](https://github.com/dotnet/msbuild/issues/14971) [#14979](https://github.com/dotnet/msbuild/issues/14979) Settle diagnostic scope, clear false positives, update msbuild docs [size L]
- [ ] Adopt in MSBuild, VMR and Arcade, improving detections from what real usage surfaces [size M]
- [ ] Ship publicly with `Microsoft.Build.Framework` [size XS] [Milestone: version 18.12]

## Stakeholders

Task authors in dotnet/sdk, dotnet/nuget, dotnet/roslyn, dotnet/arcade and the VMR, plus task authors referencing latest `Microsoft.Build.Framework`.

## Definition of done

A task author referencing `Microsoft.Build.Framework` gets the analyzer automatically, with a default scope that raises no new build-breaking diagnostics on non-MT tasks, documented configuration to opt in to full migration analysis, no known P1 false positives, and VMR and Arcade building against it.