fix(helpers/mergedeep): fix potential cases when source or target obects can be mutated

[yurisldk]

helpers/mergeDeep has got potential cases when we can mutate target or source objects, it doesn't effected to theme for now but it could be a problem someday. Now it should cover all cases. Returned type also fixed.

Test cases added.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have checked my code and corrected any misspellings

[codecov]

Codecov Report

Base: 97.34% // Head: 97.36% // Increases project coverage by +0.02% 🎉

Coverage data is based on head (a031032) compared to base (491e2f6).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #476      +/-   ##
==========================================
+ Coverage   97.34%   97.36%   +0.02%     
==========================================
  Files         127      127              
  Lines        5349     5358       +9     
  Branches      436      441       +5     
==========================================
+ Hits         5207     5217      +10     
+ Misses        142      141       -1     

Impacted Files	Coverage Δ
src/lib/components/Flowbite/Flowbite.tsx	100.00% <100.00%> (ø)
src/lib/helpers/mergeDeep.ts	100.00% <100.00%> (+2.56%)	⬆️

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