feat(datepicker): use readonly arrays for inputs (#2773)

[reduckted]

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Part of #2773.

[peterblazejewicz]

why specific type instead of readonly modifier?

@Input() months: readonly MontViewModel[] = [];

I would tend to readonly, just wondering on the choice

[reduckted]

@peterblazejewicz why specific type instead of readonly modifier?

I just followed what was specified in the associated issue.

A proposal is to use ReadonlyArray<T> instead of T[]

[peterblazejewicz]

Tha'ts ok. I've updated the origin issue with description on the TS 3.4 improvements in regards of readonly types:
#2773 (comment)

[maxokorokov]

Left some comments

[codecov-io]

Codecov Report

Merging #3424 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3424   +/-   ##
=======================================
  Coverage   91.13%   91.13%           
=======================================
  Files          96       96           
  Lines        2797     2797           
  Branches      516      516           
=======================================
  Hits         2549     2549           
  Misses        190      190           
  Partials       58       58

Flag	Coverage Δ
#e2e	56.16% <ø> (ø)	⬆️
#unit	87.97% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
src/datepicker/jalali/ngb-calendar-persian.ts	3.22% <ø> (ø)	⬆️
src/datepicker/ngb-calendar.ts	94.33% <ø> (ø)	⬆️
src/datepicker/hijri/ngb-calendar-hijri.ts	88.23% <ø> (ø)	⬆️
src/datepicker/hebrew/ngb-calendar-hebrew.ts	70.96% <ø> (ø)	⬆️

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[maxokorokov]

LGTM, thanks!