Add registry value for easy detection (PowerShell#6094)

Add registry value for easy detection. For example, the presence of a single key of 6.0.* x64. This is much faster than MSI detection.

Note there are currently two GUIDs, one for 6.0.* x64 and one for 6.0.* x86.

fixes PowerShell#6090

assets/Product.wxs

@@ -108,8 +108,13 @@
             <Component Id="ProductVersionFolder" Guid="{e1a7f05e-0cd6-4227-80a8-e4fb311f045c}">
               <CreateFolder/>
             </Component>
-            <!-- register ourselves in application registry so can be started using just Win+R `pwsh.exe` -->
             <Component Id="RegistryEntries" Guid="{402e52f7-baf8-489d-af21-f756a6ca3530}">
+                <!-- create key for easy detection of a particular version of a powershell core package 
+                     The upgrade code is used in the key because it will change when we allow SxS       -->
+                <RegistryKey Root="HKLM" Key="Software\Microsoft\PowerShellCore\InstalledVersions\$(var.UpgradeCode)" ForceCreateOnInstall="yes" ForceDeleteOnUninstall="yes">
+                    <RegistryValue Type="string" Value="$(var.ProductSemanticVersion)"/>
+                </RegistryKey>
+                <!-- register ourselves in application registry so can be started using just Win+R `pwsh.exe` -->
                 <RegistryKey Root="HKLM" Key="Software\Microsoft\Windows\CurrentVersion\App Paths\pwsh.exe" ForceCreateOnInstall="yes" ForceDeleteOnUninstall="yes">
                     <RegistryValue Type="string" Value="[$(var.ProductVersionWithName)]pwsh.exe"/>
                 </RegistryKey>