browser: implement log function for the formatters option

[KatelynKim]

The non-browser version of pino takes in a formatters option, an object that may include a log function for formatting log objects. We want to replicate this behavior in the browser pino.

Usage:

const logger = pino({
  browser: {
    formatters: {
      log (obj) {
        return {...obj, newField: 'awesome!'}
      }
    }
  }
})

[github-actions]

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