Better validation of resources with invalid names

Raised from https://github.com/Azure/bicep/pull/1800#discussion_r594791321

We should be able to say definitively that the following will not work as resource names:

```bicep
resource vmExt 'Microsoft.Compute/virtualMachines/extensions@2020-06-01' = {
name: 'myExt/'
location: 'eastus'
}

resource attachment 'Microsoft.ApiManagement/service/apis/issues/attachments@2019-01-01' = {
name: '///'
}
```

Start by reproducing the two resource-name examples in the issue and inspect the existing resource-name validation entry point. Done means both invalid names are definitively rejected, with coverage demonstrating the expected validation behavior.

Written by the indexing model from the issue text.