fix: buffer is not defined in browser

[flyfishzy]

Background

Buffer is an API in Node.js and does not exist in browsers. When upgrading to webpack 5, webpack will no longer automatically bundles core node modules (buffer/process etc.). Which will cause the Buffer is not defined problems.
mqttjs/MQTT.js#1412
mqttjs/MQTT.js#1338

Solutions

1. Although we can add polyfills by modifying the webpack.config.js file. But there are also some scenarios where the webpack.config.js cannot be modified (such as the Openharmony development environment I‘m using).
2. The Node.js doc recommends to explicitly import or require 'buffer':

While the Buffer class is available within the global scope, it is still recommended to explicitly reference it via an import or require statement.
https://nodejs.org/api/buffer.html

After that, just insall 'buffer' can fix this problem.

npm install buffer

[ralrom]

Shouldn't the mqtt-packet dependency also be updated to v8.0.0?

Otherwise we still have the same problem when trying to use MQTT.js in the browser.