fix: add default export

[robertsLando]

This adds default export to make it work on nodejs too so this module can be imported in the same way in all environments (browser and nodejs):

import mqtt from 'mqtt'

Fixes #1738

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (365e7a0) 81.03% compared to head (a8ffa7c) 81.08%.

❗ Current head a8ffa7c differs from pull request most recent head fe2a995. Consider uploading reports for the commit fe2a995 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1740      +/-   ##
==========================================
+ Coverage   81.03%   81.08%   +0.04%     
==========================================
  Files          21       22       +1     
  Lines        1366     1369       +3     
  Branches      323      323              
==========================================
+ Hits         1107     1110       +3     
  Misses        182      182              
  Partials       77       77              

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