Add PowerShell variables corresponding to platform

This deprecates the Microsoft.PowerShell.Platform module and ensure that
IsLinux/OSX/Windows/Core variables are defined all throughout PowerShell.

src/System.Management.Automation/engine/InitialSessionState.cs

@@ -5101,6 +5101,32 @@ end
                 RemotingErrorIdStrings.PSSessionAppName,
                 ScopedItemOptions.None),
             // End: Variables which control remoting behavior
+
+            #region Platform
+            new SessionStateVariableEntry(
+                SpecialVariables.IsLinux,
+                Platform.IsLinux,
+                String.Empty,
+                ScopedItemOptions.ReadOnly | ScopedItemOptions.AllScope),
+
+            new SessionStateVariableEntry(
+                SpecialVariables.IsOSX,
+                Platform.IsOSX,
+                String.Empty,
+                ScopedItemOptions.ReadOnly | ScopedItemOptions.AllScope),
+
+            new SessionStateVariableEntry(
+                SpecialVariables.IsWindows,
+                Platform.IsWindows,
+                String.Empty,
+                ScopedItemOptions.ReadOnly | ScopedItemOptions.AllScope),
+
+            new SessionStateVariableEntry(
+                SpecialVariables.IsCore,
+                Platform.IsCore,
+                String.Empty,
+                ScopedItemOptions.ReadOnly | ScopedItemOptions.AllScope),
+            #endregion
         };
 
         /// <summary>

src/System.Management.Automation/engine/SpecialVariables.cs

@@ -156,6 +156,20 @@ namespace System.Management.Automation
         internal const string PSModuleAutoLoading = "PSModuleAutoLoadingPreference";
         internal static VariablePath PSModuleAutoLoadingPreferenceVarPath = new VariablePath("global:" + PSModuleAutoLoading);
 
+        #region Platform Variables
+        internal const string IsLinux = "IsLinux";
+        internal static VariablePath IsLinuxPath = new VariablePath("IsLinux");
+
+        internal const string IsOSX = "IsOSX";
+        internal static VariablePath IsOSXPath = new VariablePath("IsOSX");
+
+        internal const string IsWindows = "IsWindows";
+        internal static VariablePath IsWindowsPath = new VariablePath("IsWindows");
+
+        internal const string IsCore = "IsCore";
+        internal static VariablePath IsCorePath = new VariablePath("IsCore");
+
+        #endregion
         #region Preference Variables
 
         internal const string DebugPreference = "DebugPreference";