Missing subnet() and isReplicated() functions in new Internet Computer module

[Demali-876]

I'd like to request keeping the subnet() and isReplicated() functions in the new InternetComputer module.

Looking at the implementation in ExperimentalInternetComputer, they're simply wrappers around primitive functions that already exist:

public let subnet : () -> Principal = Prim.canisterSubnet;
public let isReplicated : () -> Bool = Prim.isReplicatedExecution;

These functions provide valuable functionality with minimal maintenance overhead since they directly expose existing primitives. They're important for determining a canister's subnet context and execution mode, which i think is pretty useful. I also think this could be even more useful if in the future subnet information is made available through the management canister. This would create a pathway for canisters to programmatically determine and react to subnet state which i think is pretty cool.

Since the underlying primitives already exist, exposing them through the standard library would maintain backward compatibility while giving developers access to these important system details.

[ggreif]

Yeah, this is #232 and #213. We cannot have those unless we bump dfx to 0.26.0 (in beta), which will have them accessible for users.