fix: Fixes calendar jumping when switching months

[pan-kot]

Description

The PR introduces placeholder dates to Calendar, Date picker, and Date range picker components such that each calendar always has exactly 6 rows of dates which ensures its size stays constant when user switches between months.

Rel: [llayAeObY9Ic], AWSUI-60266, #3140

How has this been tested?

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.43%. Comparing base (1f1d269) to head (0fc25bf).
Report is 30 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3178      +/-   ##
==========================================
+ Coverage   96.40%   96.43%   +0.02%     
==========================================
  Files         784      788       +4     
  Lines       22143    22256     +113     
  Branches     7594     7233     -361     
==========================================
+ Hits        21348    21463     +115     
+ Misses        788      786       -2     
  Partials        7        7              

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[pan-kot]

Using data- attribute instead of :nth-child() to exclude rows with placeholder dates only. That is necessary for the backward-compatibility, to ensure the indices of the monthly dates do not change.

[pan-kot]

The calendar abstraction helps calculating indices and styles for calendar grid, which is non-trivial due to presence of placeholder dates that are treated differently.

Ideally, the same abstraction should be reused between Calendar, and Date range picker components. However, those are currently using very different implementations and only one supports the year calendar. We can reuse the code between the components as soon we update those to use the same calendar element implementation.