Move to slnx solution format (#2221)

* Move to slnx solution format

* anchor build script to new slnx file

* do not build build.csproj during release

* reanchor tests to slnx

Author: Mpdreamz

build/BuildInformation.fs

@@ -18,7 +18,7 @@ type Paths =
     static member Root =
         let mutable dir = DirectoryInfo(".")
         let mutable rooted = false
-        while not(rooted) && dir.GetFiles("*.sln").Length = 0 do
+        while not(rooted) && dir.GetFiles("*.slnx").Length = 0 do
             match dir.Parent with
             | NonNull parent -> dir <- parent
             | _ -> rooted <- true