feat(datepicker): respect first day of week in current locale

[valorkin]

PR Checklist

Before creating new PR, please take a look at checklist below to make sure that you've done everything that needs to be done before we can merge it.

• read and followed the CONTRIBUTING.md guide.
• built and tested the changes locally.
• added/updated tests.
• added/updated API documentation.
• added/updated demos.

[YevheniiaMazur]

Tested, looks good

[codecov]

Codecov Report

Merging #2970 into development will decrease coverage by 0.09%.
The diff coverage is 35.29%.

@@              Coverage Diff               @@
##           development    #2970     +/-   ##
==============================================
- Coverage        65.78%   65.68%   -0.1%     
==============================================
  Files              207      207             
  Lines             5606     5619     +13     
  Branches           983      985      +2     
==============================================
+ Hits              3688     3691      +3     
- Misses            1656     1666     +10     
  Partials           262      262

Impacted Files	Coverage Δ
src/datepicker/utils/bs-calendar-utils.ts	34.78% <25%> (-6.4%)	⬇️
src/datepicker/engine/format-days-calendar.ts	35.71% <42.85%> (-8.74%)	⬇️
src/datepicker/reducer/bs-datepicker.reducer.ts	13.63% <50%> (+0.67%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update e16efee...24e707b. Read the comment docs.