Eliminate the need to provide index in DragAndDrop

[TomasEng]

Description

Now, consumers have to provide an index to the DragAndDrop items, like this:

<DragAndDrop.List>
    <DragAndDrop.ListItem index={0}/>
    <DragAndDrop.ListItem index={1}/>
    <DragAndDrop.ListItem index={2}/>
    ...
</DragAndDrop.List>

Providing the wrong index will lead to buggy behaviour, so this is not ideal. This pull request introduces a findPositionInList function that is used to find the correct index from within the component using the DOM model. It is only called when the user drags something, so there should not be any risk of it being called before the DOM is fully rendered.

In order to avoid selector collisions, I have also added a useDomSelectors hook that generates unique ids and class names that are used by findPositionInList.

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)
• Cypress tests run green locally (https://github.com/Altinn/altinn-studio/tree/master/frontend/testing/cypress#run-altinn-studio-tests)

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (0a2cc67) 80.85% compared to head (85be382) 80.92%.
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #11252      +/-   ##
==========================================
+ Coverage   80.85%   80.92%   +0.06%     
==========================================
  Files         751      753       +2     
  Lines       11940    11982      +42     
  Branches     1597     1598       +1     
==========================================
+ Hits         9654     9696      +42     
  Misses       2038     2038              
  Partials      248      248              

Files	Coverage Δ
...ts/dragAndDrop/DragAndDropList/DragAndDropList.tsx	81.81% <100.00%> (+1.81%)	⬆️
.../DragAndDropListItem/DragAndDropListItemContext.ts	100.00% <100.00%> (ø)
...ndDrop/DragAndDropProvider/DragAndDropProvider.tsx	100.00% <100.00%> (ø)
...Drop/DragAndDropProvider/DragAndDropRootContext.ts	100.00% <ø> (ø)
...rc/components/dragAndDrop/hooks/useDomSelectors.ts	100.00% <100.00%> (ø)
...hared/src/components/dragAndDrop/utils/domUtils.ts	100.00% <100.00%> (ø)
...d/packages/ux-editor/src/containers/DesignView.tsx	88.23% <ø> (ø)
...ndDrop/DragAndDropListItem/DragAndDropListItem.tsx	78.94% <85.71%> (+1.80%)	⬆️

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[framitdavid]

LGTM!👏 🎖️