feat: add source control metadata to packages

[dbjorge]

Details

This PR adds source-linking metadata to each package using SourceLink, which allows IDEs to find the specific source commit/line during debugging that involves the packages. It does this by following the sourcelink docs to add a development-time (PrivateAssets="All") reference on Microsoft.SourceLink.GitHub.

The selenium package already had a (unnecessarily verbose) version of this in place; this PR updates all 3 packages to consistently use the recommended form of sourcelink reference.

You can verify the results of this by building, opening up the resulting .nupkg files, and finding a line like this embedded automatically into the included .nuspec file:

<repository type="git" url="https://github.com/dequelabs/axe-core-nuget" commit="b7b933324ab5201a361ef0a3b49a5fcae0494d19" />

Closes Issue: n/a

[AdnoC]

• reviewed for security