Added basic implementation to toggle the placeholder text

[navdeepgill14]

References

Closes #15138

Code changes

1. Introduced a global state variable isPlaceholderVisible in widget.ts to manage the visibility of the placeholder text.
2. Implemented a function toggleGlobalPlaceholderVisibility in widget.ts to toggle the visibility state.
3. Added conditional rendering logic in the cell widget rendering method in widget.ts to show/hide the placeholder based on the isPlaceholderVisible state.

User-facing changes

1. Users will see a toggle button in the notebook toolbar. Clicking this button will show or hide the placeholder text in the notebook cells.
2. This provides users with the flexibility to choose whether they want to see the placeholder text or not.

Backwards-incompatible changes

None. The changes introduced are compatible with the existing JupyterLab public APIs and do not break any existing functionality.

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[krassowski]

Thanks for starting this. We cannot add a new toolbar button for every setting, this should be rewritten to be a setting in Notebook schema instead. Please do ask if you need pointers.

[navdeepgill14]

Thank you for the feedback, @krassowski . I understand the concern about adding too many toolbar buttons. I'll work on moving this functionality to the Notebook schema as a setting. I'm relatively new to this, so I'd appreciate any pointers or guidance you can provide on how to best implement this in the Notebook schema. Thanks for your patience and support!

[krassowski]

A recent PR that implemented a new Notebook setting was #13944 - have a look at which files were modified in there to thread inputHistoryScope down.

[krassowski]

@navdeepgill14 are you still interested in working on this?