Implementing built in date formatting

[demontiejr]

Currently, the number.Format method doesn't take into consideration the formatting code. This diff implements formatting for dates based on the code.
The implementation doesn't look at the cell data type because in cases of date it's not being populated by Excel, so it's always empty.

[codecov]

Codecov Report

Merging #51 into master will decrease coverage by 0.38%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master      #51      +/-   ##
==========================================
- Coverage   63.49%   63.10%   -0.39%     
==========================================
  Files         100      100              
  Lines        4005     3288     -717     
==========================================
- Hits         2543     2075     -468     
+ Misses       1417     1169     -248     
+ Partials       45       44       -1     

Impacted Files	Coverage Δ
internal/number_format/types.go	71.42% <75.00%> (+1.05%)	⬆️
internal/number_format/indexed.go	100.00% <100.00%> (ø)
internal/ml/primitives/gradient_type.go	80.00% <0.00%> (-4.62%)	⬇️
conditional.go	3.44% <0.00%> (-4.25%)	⬇️
internal/color/color.go	81.81% <0.00%> (-3.90%)	⬇️
format/conditional/rule/databar.go	64.58% <0.00%> (-3.84%)	⬇️
filters.go	2.56% <0.00%> (-3.82%)	⬇️
col.go	66.66% <0.00%> (-3.34%)	⬇️
xlsx.go	80.00% <0.00%> (-3.34%)	⬇️
internal/validator/validator.go	89.47% <0.00%> (-3.12%)	⬇️
... and 92 more

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[plandem]

I can't approve these changes, since it changes defaults from the official docs for XLSX. I probably will use this PR as a reference when I will have time to work on it.