Add dutch language support

[diegonuja]

Hi,

I'd like to add support for Dutch.

Thanks!

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (a fix or feature that would cause existing functionality to change)

Checklist

• Directed the PR to develop branch and not the master branch
• Wrote additional tests, if needed
• All tests have passed
• I have updated the documentation accordingly, if needed.

[codecov-commenter]

Codecov Report

Merging #114 (6f49fcb) into develop (35dab1e) will decrease coverage by 0.49%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #114      +/-   ##
===========================================
- Coverage    93.93%   93.44%   -0.50%     
===========================================
  Files           29       31       +2     
  Lines         1187     1266      +79     
===========================================
+ Hits          1115     1183      +68     
- Misses          72       83      +11     

Impacted Files	Coverage Δ
lib/src/locale/availableLocales.dart	98.18% <100.00%> (+0.06%)	⬆️
lib/src/locale/nlLocale.dart	100.00% <100.00%> (ø)
lib/src/locale/ukLocale.dart	77.08% <0.00%> (ø)

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[jama5262]

Please make sure you've added the nl in the docs here https://github.com/jama5262/jiffy/tree/v4.1.0/doc#locale-support