Modernize build

[richardszalay]

While putting together #20, it became painfully clear that the existing build system needs some serious upgrades.

There are a few major issues with the build system today:

• It relies on Psake, which is either no longer supported or at least has very strong opinions on MSBuild selection that don't work with modern .NET SDK setups
• It only runs on Windows
• It relies on both Appveyer (Windows) and Travis (Linux) that are connected in a very unconventional (and confusing) way

The limitations above were deemed acceptable in order to continue supporting PowerShell 3.0, but I don't think that's necessary anymore.

The proposal for the new build system:

• [x] Update libraries to target netstandard-2.0 and PowerShellStandard.Library@5.1.0
• [x] Remove PSake and replace it with an MSBuild-based approach (TODO: How will the Pester tests work?)
• [ ] Port CI to GitHub Actions and run on Windows + Linux (+ macOS?)

Additional reading:

• Writing Portable Modules
• Depending on the right PowerShell NuGet package in your .NET project

[richardszalay]

Completed everything except the CI integration. Will leave this issue open until I get to the GH Actions bit.