feat(browser): add logEventToObject helper for transmit.send

[mcollina]

Summary

• Add logEventToObject helper function that converts a LogEvent to a log object similar to what Node.js transports receive
• This makes it easier to send formatted logs to upstream logging systems (like DataDog) from transmit.send
• The function merges child bindings into a single object, extracts the merging object from the first message, and formats the message string with arguments

Usage

const pino = require('pino')
const { logEventToObject } = require('pino')

const logger = pino({
  browser: {
    transmit: {
      send(level, logEvent) {
        const logObject = logEventToObject(logEvent, { messageKey: 'msg' })
        // Send logObject to your logging service
        // { time, level, ...bindings, msg }
      }
    }
  }
})

Test plan

• Added unit tests for logEventToObject function
• Tests cover basic conversion, binding merging, message formatting, custom messageKey, and integration with transmit.send
• Verified TypeScript definitions compile correctly
• All browser-transmit tests pass

Closes #1556