[source-build-externals] Consolidate external repo version values into Versions.props

[mthalman]

Package version properties are currently defined in the individual repo project files (e.g. https://github.com/dotnet/source-build-externals/blob/8f12bdccd3a0a02ed1e0714b99bbe25e7ef8e04e/repo-projects/humanizer.proj#L7, https://github.com/dotnet/source-build-externals/blob/8f12bdccd3a0a02ed1e0714b99bbe25e7ef8e04e/repo-projects/spectre-console.proj#L6C1-L6C1). These could be consolidated into the eng/Versions.props file to follow the existing Arcade pattern. See dotnet/source-build-externals#248 (comment)

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.