scope property in deployment() function

**Is your feature request related to a problem? Please describe.**
We have function like subscription() and soon to have managementGroup(). There are a few resources that allow to deployed at both subscription and management group scopes. The difference when these resources are deployed at these two scopes are minor. For example the main differences I have seen are:
- cannot deploy resource group at mg level
- policy definition and assignment names needs to be maximum 24 characters at management group level (at sub level it is higher number). No idea why this limit exists but it is there.
With that said if you want to create one bicep solution that can deploy the applicable resources to both scopes it is good that you can use managementGroup() or subscription() function depending on the scope the deployment happens.

**Describe the solution you'd like**
the solution would be something like:

```bicep
var someVar = deployment().scope =~ 'subscription' ? subscription().id : managementGroup().id
```

Start by reading the existing deployment(), subscription(), and managementGroup() function handling and their related tests. Determine how deployment scope is represented and validated, then define and test behavior for selecting the applicable scope so the example expression works at both subscription and management-group deployments.

Written by the indexing model from the issue text.