Add PowerShell as a valid value for FUNCTIONS_WORKER_RUNTIME

This PR updates the table listing the function app settings to add `powershell` as a valid value for the `FUNCTIONS_WORKER_RUNTIME` setting. This reflects that PowerShell is now a supported language for Azure Functions.

Author: johndowns

articles/azure-functions/functions-infrastructure-as-code.md

@@ -135,7 +135,7 @@ A function app must include these application settings：
 |------------------------------|-------------------------------------------------------------------------------------------|---------------------------------------|
 | AzureWebJobsStorage          | A connection string to a storage account that the Functions runtime for internal queueing | See [Storage account](#storage)       |
 | FUNCTIONS_EXTENSION_VERSION  | The version of the Azure Functions runtime                                                | `~2`                                  |
-| FUNCTIONS_WORKER_RUNTIME     | The language stack to be used for functions in this app                                   | `dotnet`, `node`, `java`, or `python` |
+| FUNCTIONS_WORKER_RUNTIME     | The language stack to be used for functions in this app                                   | `dotnet`, `node`, `java`, `python`, or `powershell` |
 | WEBSITE_NODE_DEFAULT_VERSION | Only needed if using the `node` language stack, specifies the version to use              | `10.14.1`                             |
 
 These properties are specified in the `appSettings` collection in the `siteConfig` property: