Fixing several issues with the new Datepicker and making behavior closer to the old one

[bjosttveit]

Description

Fixes:

• DateInput is able to parse any ISO string, not just the particular format we save
• DateInput automatically fills inn separator characters
  • This is similar to how the old one worked
  • Now you can only type numbers, and no more than required
  • To achieve this, I used the PatternFormat which is also used for number-formatting in the Input-component. I modify the raw format to have fixed length for day, month, and year to make this work. This also makes the error message less confusing, see: https://altinn.slack.com/archives/C02EJ9HKQA3/p1729692095041289?thread_ts=1729512493.045959&cid=C02EJ9HKQA3
    
    Now it looks like this instead with the default format for nb:
    
• DayPicker: changing month using arrows or dropdowns now works
• DayPicker: clicking the small arrows (chevron) in dropdown now works as expected
• DayPicker: Dropdowns now scroll to the selected value, instead of showing the year 2124
  • This required a workaround because the Combobox component does not have any support for this.

Other changes

I refactored a few things, for example moving the calendar button outside of the input component and structuring the dialog a bit differently.

Related Issue(s)

• closes Datepicker only works with manual input #2608
• closes DatePicker v4.9: Flere problemer ved valg fra nedtrekksmenyer #2637

Verification/QA

• Manual functionality testing
  • I have tested these changes manually
  • Creator of the original issue (or service owner) has been contacted for manual testing (or will be contacted when released in alpha)
  • No testing done/necessary
• Automated tests
  • Unit test(s) have been added/updated
  • Cypress E2E test(s) have been added/updated
  • No automatic tests are needed here (no functional changes/additions)
  • I want someone to help me make some tests
• UU/WCAG (follow these guidelines until we have our own)
  • I have tested with a screen reader/keyboard navigation/automated wcag validator
  • No testing done/necessary (no DOM/visual changes)
  • I want someone to help me perform accessibility testing
• User documentation @ altinn-studio-docs
  • Has been added/updated
  • No functionality has been changed/added, so no documentation is needed
  • I will do that later/have created an issue
• Support in Altinn Studio
  • Issue(s) created for support in Studio
  • This change/feature does not require any changes to Altinn Studio
• Sprint board
  • The original issue (or this PR itself) has been added to the Team Apps project and to the current sprint board
  • I don't have permissions to do that, please help me out
• Labels
  • I have added a kind/* label to this PR for proper release notes grouping
  • I don't have permissions to add labels, please help me out

[sonarcloud]

Quality Gate passed

Issues
6 New issues
0 Accepted issues

Measures
0 Security Hotspots
85.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[paal2707]

Veldig bra!