feat: strongly name assemblies

[AdnoC]

Closes Issue: https://github.com/dequelabs/axe-core-nuget/issues/105

Note that it is per Microsoft's own guidance, that the private key has been added to the repo (the last paragraph on that page).

Selenium do not provide strongly named assemblies, so our selenium package is still weakly named (as you can only use strongly named dependencies from a strongly named package).

[dbjorge]

To my knowledge, there isn't really any thrilling solution to this problem:

• it's not realistic to ask users who are using strong-naming themselves to stop doing that, since that would also be a breaking change for that user
• we could publish separate Deque.Axe.Commons and Deque.Axe.Commons.StrongNamed nuget packages, but this is strongly disrecommended by the .NET docs, mostly because it makes it very unpleasant for users with a diamond dependency on us with one strong-named and one non-strong-named path
• we could copy Selenium's strategy, where they only publish a non-strong-named version to NuGet but do provide a strong-named version outside of nuget. This essentially establishes the non-strong-named version as the "default" that almost all users will use and minimizes how likely it is to end up with a diamond dependency problem, but it's much less convenient for users that need the strong-named version (example user complaint about it in the selenium repo)