fix(notebook-cell): add a slot for drag handle

[bsahitya]

Description

• With the current implementation, drag logic will be applied to the entire cell. This is causing issues with text selection, or scrolling of outputs.
• In order to prevent the entire cell component from being dragged, add a slot for drag handle where users can pass any component. They can add the drag logic to this slot.

What's included?

• Add a slot called drag-handle in the notebook cell component
• Update notebook cell story

Test Steps

• npm run storybook
• Open notebook cell story
• Make sure drag handle is rendered correctly

General Tests for Every PR

• npm run start still works.
• npm run lint passes.
• npm run stylelint passes.
• npm test passes and code coverage is not lower.
• npm run build still works.

Screenshots or link to StackBlitz/Plunker

[owilliams320]

🎉 This PR is included in version 8.20.4 🎉

The release is available on:

• v8.20.4
• GitHub release

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