Replace xml-js with fast-xml-parser

[nodaguti]

This PR migrates from xml-js to fast-xml-parser.

The motivations are:

• xml-js depends on Node.js's buffer and stream modules, which means we have to add browserify-buffer and stream packages as polyfills
  • Browser bundling nashwaan/xml-js#54
  • Can't resolve 'stream' nashwaan/xml-js#66
  • Module not found: Error: Can't resolve 'buffer'  nashwaan/xml-js#213
• xml-js's development has been inactive for these five years and many bugs remain unresolved

fast-xml-parser, by contrast, has no dependencies on Node.js modules, is actively developed, and is funded by a company. I believe fast-xml-parser is more reliable and contributes improved interoperability.

I've verified that all tests are passed and a PRE ad is properly rendered on Chromium 51 in Docker and a real device when integrated with our application.

Thank you in advance for your review.