Implement scopes for extension resources

[anthony-c-martin]

Implementation of https://github.com/Azure/bicep/blob/main/docs/spec/resource-scopes.md for extension resources.

The following syntax should be possible (using the 'scope' property to set the parent scope for an extension resource:

resource myWebsite 'Microsoft.Web/sites@2019-08-01' = {
  name: siteName
  location: resourceGroup().location
  properties: {
    <omitted>
  }
}

resource myLock 'Microsoft.Authorization/locks@2020-01-01' = {
  scope: myWebsite
  name: 'myLock'
  properties: {
    level: 'CanNotDelete'
  }
}