fix(NcFormBox): add nested NcFormBox row support

[ShGKme]

☑️ Resolves

• Based on https://github.com/nextcloud-libraries/nextcloud-vue/pull/7886
• Use case - split buttons
  • We do not have components requiring this feature at the moment
  • But there were some individual cases

🖼️ Screenshots

🏁 Checklist

• [ ] ⛑️ Tests are included or are not applicable
• [ ] 📘 Component documentation has been extended, updated or is not applicable
• [ ] 2️⃣ Backport to stable8 for maintained Vue 2 version or not applicable

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 52.17%. Comparing base (55e0b84) to head (706b4bd).

Additional details and impacted files

@@                   Coverage Diff                   @@
##           fix/NcFormBox-NcButton    #7887   +/-   ##
=======================================================
  Coverage                   52.17%   52.17%           
=======================================================
  Files                         101      101           
  Lines                        3174     3174           
  Branches                      873      872    -1     
=======================================================
  Hits                         1656     1656           
  Misses                       1271     1271           
  Partials                      247      247           

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[ShGKme]

/backport to stable8