environment() function is missing App Service endpoint

**Bicep version**
All versions

**Describe the bug**
The `environment()` function provides a `suffixes` property, with a list of a few cloud-specific suffixes for certain services. App Service is not on this list.

It would be great if I could use `environment().suffixes.appService` and have it resolve to `.azurewebsites.net` in MAC.

**Additional context**
This would be useful for a few situations:

1. Reliably predicting the host name of an App Service/Azure Functions app that hasn't yet been deployed.
2. Reproducing the behaviour of the Azure portal when you create a private DNS zone for a App Service endpoint, where it creates a DNS zone resource named `privatelink.azurewebsites.net`.

Start by locating the implementation and tests for the environment() function's suffixes property. Add coverage for an appService suffix resolving to .azurewebsites.net in MAC, then run the relevant environment() tests to verify the new value.

Written by the indexing model from the issue text.