feat: NuGet release/test workflows

[Chriztiaan]

This PR introduces three workflows:

1. Testing packages
2. Dev packages release
3. Release

Functionally the dev and proper releases are the same (and dispatched by hand). They will might differ a bit in the future especially in terms of how the package tag is derived.

Currently both releases grabs the first version from the CHANGELOG.md and use that for the package's name. For dev releases you would manually add a version to the start of the CHANGELOG.md such as 0.0.3-dev.1.

Future work will include grabbing the changelog entry for the latest version and applying it to the PackageReleaseNotes tag in the package's .csproj

Minor code changes:

1. Ensuring Windows/.NET 4.8 install properly loads the PS extension
2. Some tests which weren't behaving/too slow
3. TaskCompletionSource's needing a typing for the generic.