feat: Add subform layoutset

[lassopicasso]

Description

This PR introduces the ability for users to add subform layout sets when the feature flag subForm is enabled.

Key Changes:

• Temporary UX: Since the design is not finalized, this PR focuses solely on functionality, using a temporary user interface.
• Layout Set Selector:
  • The existing layout set selector is replaced with a combobox that provides descriptions for the layout sets.
  • If a layout set is a subform, it is labeled with the description text "Underskjema".
  • Regular layout sets do not have a description.
• New Buttons:
  • Add New Subform: Allows users to add a subform layout set. Reusing the validation hook useValidateLayoutSetName
  • Delete Selected Subform: Allows users to delete a subform. The delete button is disabled when a regular layout set is selected.
• Data Storage:
  • When a new subform is created, it is stored in layout-sets.json.
  • Regular layout sets are stored with a tasks property, while subforms are identified by a type property.

Once this PR is merged, issue #13475 can be unblocked.

Related Issue(s)

• Support adding a SubForm layoutset #13479

Verification

• Your code builds clean without any errors or warnings
• Manual testing done (required)
• Relevant automated test added (if you find this hard, leave it and we'll help out)

Documentation

• User documentation is updated with a separate linked PR in altinn-studio-docs. (if applicable)

[codecov]

Codecov Report

Attention: Patch coverage is 98.50746% with 1 line in your changes missing coverage. Please review.

Project coverage is 94.24%. Comparing base (ba935fe) to head (b864a3f).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...or/src/components/Elements/LayoutSetsContainer.tsx	93.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13632      +/-   ##
==========================================
+ Coverage   94.23%   94.24%   +0.01%     
==========================================
  Files        1528     1532       +4     
  Lines       20814    20868      +54     
  Branches     2511     2516       +5     
==========================================
+ Hits        19614    19668      +54     
  Misses        951      951              
  Partials      249      249              

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[framitdavid]

Nice work @lassopicasso! This looks good for me now. 👏

[standeren]

Some small comments to consider 🌟

[standeren]

Nice 🥳 Lets release the blocked issue 😈