Add ability to calculate number of subnets to CIDR functions

In addition to the currently [available CIDR functions](https://learn.microsoft.com/azure/azure-resource-manager/bicep/bicep-functions-cidr), I would also like to be able to calculate how many subnets could be carved out. This function would return a single integer.

**Describe the solution you'd like**

Example:

```bicep
var maxSubnets = cidrSubnetCount(network, newCIDR)
```

For `network = 10.0.0.0/16` and `newCIDR = 24`, this would return `256`.

Read the linked CIDR functions documentation first to understand the existing function behavior and naming. Use the issue's 10.0.0.0/16 and /24 example as the initial expected result, then verify that the new function returns the correct subnet count for equivalent CIDR inputs.

Written by the indexing model from the issue text.