Brought back PowerShell's AssemblyLoadContext

Removes dependency on Microsoft.Extensions.Platform.Abstractions.

Makes use of native executable from `dotnet-cli` impossible; must use
`powershell` from monad-native.

src/Microsoft.PowerShell.Linux.Host/main.cs

@@ -18,13 +18,6 @@ namespace Microsoft.PowerShell.Linux.Host
         /// </summary>
         public static void Main(string[] args)
         {
-            // create the typecatalog so they can find assemblies for a given type.
-            TypeCatalog.GenerateTypeCatalog();
-            if (System.Management.Automation.ClrHost.TypeCatalog == null)
-            {
-                Console.WriteLine("TypeCatalog generation failed!");
-            }
-
             // Custom argument parsing
             string initialScript = null;
             if (args.Length > 0)