Add managed entrance for powershell project

The powershell project's `Main` function initializes the custom
`AssemblyLoadContext` and then delegates to `UnmanagedPSEntry.Start`.
This lets us undo several changes to the `ConsoleHost` managed
entry (notably, the `Start` function signature does not have to change,
thus the `consoleFilePath` variable still exists).

src/Microsoft.PowerShell.ConsoleHost/host/msh/ManagedEntrance.cs

@@ -26,9 +26,7 @@ namespace Microsoft.PowerShell
         /// <param name="args">
         /// Command line arguments to the managed MSH
         /// </param>
-#if OPEN && CORECLR // Open PowerShell needs an actual Main entry-point
-        public static int Main(string[] args)
-#elif CORECLR
+#if CORECLR
         #pragma warning disable 1573
         public static int Start(string consoleFilePath, [MarshalAs(UnmanagedType.LPArray, ArraySubType = UnmanagedType.LPWStr, SizeParamIndex = 2)]string[] args, int argc)
         #pragma warning restore 1573
@@ -36,9 +34,6 @@ namespace Microsoft.PowerShell
         public int Start(string consoleFilePath, string[] args)
 #endif
         {
-#if OPEN && CORECLR // Open PowerShell has to set the ALC here, since we don't own the native host
-            PowerShellAssemblyLoadContextInitializer.SetPowerShellAssemblyLoadContext(string.Empty);
-#endif
             System.Management.Automation.Runspaces.EarlyStartup.Init();
 
             // Set ETW activity Id
@@ -74,9 +69,7 @@ namespace Microsoft.PowerShell
                 //      PSSnapInException will cause the control to return back to the native code
                 //      and stuff the EXCEPINFO field with the message of the exception.
                 //      The native code will print this out and exit the process.
-#if !OPEN // consoleFilePath is not available in Open PowerShell
                 if (string.IsNullOrEmpty(consoleFilePath))
-#endif
                 {
 #if DEBUG
                     // Special switches for debug mode to allow self-hosting on InitialSessionState instead
@@ -103,14 +96,12 @@ namespace Microsoft.PowerShell
                 configuration = null;
 #endif
                 }
-#if !OPEN // consoleFilePath is not available in Open PowerShell
                 else
                 {
                     //TODO : Deprecate RunspaceConfiguration and use InitialSessionState
                     configuration =
                         RunspaceConfigForSingleShell.Create(consoleFilePath, out warning);
                 }
-#endif
                 int exitCode = 0;
                 try
                 {