fix(calendar): use last defined week day to determine week number

[yggg]

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• I read and followed the CONTRIBUTING.md guide.
• I read and followed the New Feature Checklist guide.

Short description of what this resolves:

[codecov]

Codecov Report

Merging #1969 into master will increase coverage by 0.02%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #1969      +/-   ##
==========================================
+ Coverage   84.08%   84.11%   +0.02%     
==========================================
  Files         235      235              
  Lines        7793     7794       +1     
  Branches      681      681              
==========================================
+ Hits         6553     6556       +3     
+ Misses       1041     1039       -2     
  Partials      199      199

Impacted Files	Coverage Δ
...lendar-day-picker/calendar-day-picker.component.ts	100% <ø> (ø)	⬆️
...r-week-number/nb-calendar-week-number.component.ts	64.7% <0%> (-4.05%)	⬇️
...rc/framework/theme/components/menu/menu.service.ts	88.02% <0%> (+0.7%)	⬆️
.../framework/theme/components/menu/menu.component.ts	78.04% <0%> (+1.62%)	⬆️