style: Rename subForm to subform

[lassopicasso]

Description

Renaming folders, files and code from subForm/SubForm to subform/Subform.

Related Issue(s)

• Replace subForm with subform #13896

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 97.14286% with 1 line in your changes missing coverage. Please review.

Project coverage is 95.13%. Comparing base (4f52f21) to head (f749397).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...tend/packages/ux-editor/src/data/formItemConfig.ts	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #13903   +/-   ##
=======================================
  Coverage   95.13%   95.13%           
=======================================
  Files        1673     1673           
  Lines       22189    22189           
  Branches     2608     2608           
=======================================
  Hits        21109    21109           
  Misses        834      834           
  Partials      246      246           

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[Jondyr]

👍

[JamalAlabdullah]

Tested ok in dev 🙂