fix: fixing and exporting mergeMessages()

[pedromartello]

Fixed and exported the internal function mergeMessages() and refactored unit-tests to cover the new behavior. Previously this method would return an object identical to the one passed in (or an object version of an array passed in) and ignore/clobber any messages previously supplied. The new version fixes this function to actually merge new messages with existing ones.

The method was never exported outside the package and the only internal use was in the configure() method which is refactored in this PR to use lodash.merge() directly.

These changes support OEP-65 and the related work in the repository frontend-app-shell which uses the Piral framework to federate MFEs.

With existing MFE's which are deployed and run independently, there is never a need to merge messages into frontend-platform after initialization is completed. However, with federated MFEs, they need the ability to append new messages to those already loaded. This change supports this requirement.

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[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.03% 🎉

Comparison is base (26001c9) 83.05% compared to head (1a4203c) 83.08%.
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #550      +/-   ##
==========================================
+ Coverage   83.05%   83.08%   +0.03%     
==========================================
  Files          40       40              
  Lines        1062     1064       +2     
  Branches      195      194       -1     
==========================================
+ Hits          882      884       +2     
  Misses        168      168              
  Partials       12       12              

Files Changed	Coverage Δ
src/i18n/lib.js	98.24% <100.00%> (+0.06%)	⬆️

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[arbrandes]

Nice! It looks like that utility function now actually has a utility, now. :)

Tested, and has no ill effects in existing MFEs. (Not that any of them actually used this function, AFAICT.)

[openedx-webhooks]

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