fix: add missing export of UniqueMessageIdProvider and DefaultMessageIdProvider

[ogis-ksuzuki]

I tried to build the source code using the UniqueMessageIdProvider with webpack, but it could not be called correctly.

for example)

import * as mqtt from 'mqtt';
let provider = new mqtt.UniqueMessageIdProvider();

The reason is that in the webpack build, "browser" field of package.json "./mqtt.js": "./lib/connect/index.js" causes the entry point to be "./lib/connect/index.js" and that "index.js" does not contain an export of UniqueMessageIdProvider.

In case of node build, './mqtt.js' is referenced and 'mqtt.js' exports UniqueMessageIdProvider.
Same for DefaultMessageIdProvider.

I added a DefaultMessageIdProvider and UniqueMessageIdProvider export to "index.js" and confirmed that the issue was resolved and the unit tests passed.

[robertsLando]

@ogis-ksuzuki Could you fix conflicts please?

[ogis-ksuzuki]

@robertsLando Resolved merge conflict.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.04 🎉

Comparison is base (9e3d8a3) 86.18% compared to head (2baaf32) 86.22%.

❗ Current head 2baaf32 differs from pull request most recent head f496481. Consider uploading reports for the commit f496481 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1572      +/-   ##
==========================================
+ Coverage   86.18%   86.22%   +0.04%     
==========================================
  Files          13       13              
  Lines        1310     1314       +4     
==========================================
+ Hits         1129     1133       +4     
  Misses        181      181              

Impacted Files	Coverage Δ
lib/connect/index.js	89.53% <100.00%> (+0.51%)	⬆️

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