fix: type of child logger with custom levels

[UndefinedBehaviour]

fixes child logger types so that stuff like this doesn't error anymore

const parentLogger = pino({
  customLevels: { myLevel: 90 },
  onChild: (child) => { const a = child.myLevel; }
}, process.stdout);
parentLogger.onChild = (child) => { child.myLevel(''); };

[mcollina]

lgtm