Context info for what-if deployments

Is there any way for a deployment to know whether or not it's a what-if deployment in each deployment context?

For example, if such global deployment-time context information is exposed in `deployment().metadata` each time, you can implement workarounds for the `reference()` limitation like #157 more easily:

```bicep
var vdpoolName = 'desktop1'
var vdvmName = 'vm1'
var whatif = deployment().?metadata.?whatif

// Specify the older API version to get registrationInfo https://github.com/Azure/bicep/issues/6105
resource vdpool 'Microsoft.DesktopVirtualization/hostPools@2021-01-14-preview' existing = {
name: vdpoolName
}

// Avoid reference() in params when it's a what-if deployment
module vdvm 'vdvm.bicep' = {
name: 'vdvm_${deployment().name}'
params: {
vdvmName: vdvmName
registrationInfoToken: whatif ? 'a-token' : vdpool.properties.registrationInfo.token
// Other module parameters...
}
}
```

It would be nice if the ARM infrastructure or tooling could provide this feature. Currently I must have an additional boolean parameter `whatif` and manually enable it when I run a what-if deployment.

Start by reviewing how ARM what-if deployments and Bicep's deployment().metadata are represented, along with the limitation discussed in #157. Determine whether the ARM infrastructure or tooling should expose a what-if indicator, then validate the behavior against the provided deployment example and the existing manual boolean workaround.

Written by the indexing model from the issue text.