Convert FAKE script to project

[schauerte]

I wanted to test out some performance improvements but had a very hard time trying to get the code running. So I took the (probably preferred) way to convert the build-script into a .NET project.

This should eliminate some of the problems people have when checking out FSharp.Data and prepares the way to easily upgrade to .NET 7/8 in the future.

On my machine (mac-os), some of the http-tests are failing, due to issues with the locally started web-server. I hope this was not caused by my changes (but since I wasn't able to run the tests without, I can't verify).