Fix: Prevent duplicate file renaming in sidebar

[kammeows]

Fixes #3454 and #3456

Changes:

• Imported useSelector from react-redux to access the global files state from the Redux store to read the current list of files and folders in the editor
• Added a function named checkDuplicate that checks if there are any sibling files with same name in the parent folder and returns a boolean value.

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• is descriptively named and links to an issue number, i.e. Fixes #123

[kammeows]

@raclim Currently, if a user tries to rename a file to a name that already exists in the same directory, the name simply reverts to the original without any visual feedback. No error message or dialog is shown.
Please let me know if a warning message, inline error text, or a dialog would be preferred to inform the user when renaming isn’t allowed.

[raclim]

Thanks for finding these issues and suggesting these fixes for them! I really like the renaming on double-click implementation!

In terms of giving some sort of visual feedback or warning—I think maybe we can try what you currently have here for now and see how folks respond. Thanks so much again for your work on this!