Detect unused permissions in Snaps CLI

[Mrtenz]

We should add a feature to the Snaps CLI to detect unused permissions, and log a warning if a permission is requested from the manifest but not used.

• For exports-related permissions (onRpcRequest, onTransaction, etc.) we can hook into the eval functionality to detect which exports the Snap has.
• To detect other permissions, we may need to do some AST parsing, detecting if certain methods are used, and so on, depending on complexity.
• Ideally there's some way to ignore the warnings as well (through a config option) in case of false positives.