Apply loglevels in the environment

When switching to the "official" `Logging` package, we lost the ability to easily set loglevels? Like `export LOGLEVEL=trace`.

We should fix that and have Macro configure the logging system if such env vars are found.

Boilerplate:
```swift
LoggingSystem.bootstrap { label in
  var handler = StreamLogHandler.standardOutput(label: label)
  handler.logLevel = .trace
  return handler
}
```