Added dateMapper with protection against invalid value passes as valid.

[Siemienik]

resolves: #58

This PR adds dateMapper, which transform string into Date Object:

TODO:

1. Create file src/mappers/dateMapper.ts with function matches to type ValueMapper<Date>. Which parse value string into Date.
2. reexport it in src/mappers/index.ts
3. Prove that it works in unit test (some present dates, some future, some past & some invalid)
4. Describe it on https://siemienik.com/docs/xlsx-import/#mappers : PR

[codecov]

Codecov Report

Merging #192 (cd80ce1) into master (b552554) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #192   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           54        55    +1     
  Lines         1061      1063    +2     
  Branches       134       154   +20     
=========================================
+ Hits          1061      1063    +2     

Flag	Coverage Δ
xlsx-import	100.00% <100.00%> (ø)
xlsx-renderer	100.00% <ø> (ø)

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Impacted Files	Coverage Δ
packages/xlsx-import/src/mappers/dateMapper.ts	100.00% <100.00%> (ø)
packages/xlsx-import/src/mappers/index.ts	100.00% <100.00%> (ø)

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[sonarcloud]

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0.0% Duplication