feat: extended logging support

[kukhariev]

• Built-in logger now support severity levels:

  app.use(
    '/files',
    uploadx({
      directory: 'upload',
      logLevel: <LogLevel>process.env.LOG_LEVEL || 'info'
    })
  );

• Added support for custom loggers:

  const logger = someLoggerLib.createLogger({
    level: process.env.LOG_LEVEL || 'info',
  });
  
  app.use(
    '/upload',
    uploadx({
      directory: 'upload',
      logger: logger
    })
  );