fix: properly handle VariableNode deletion in Lexical

[loicguillois]

This pull request includes several updates to the useVariablePlugin function in the frontend/src/components/RichEditor/useVariablePlugin.ts file to improve whitespace handling around VariableNode elements. The most important changes include adding a new utility function for cleaning up whitespace, updating the node insertion logic, and modifying the mutation listener.

Whitespace handling improvements:

• Added the cleanUpWhitespace function to ensure proper whitespace handling around VariableNode elements.
• Updated the isWhitespaceBefore and isWhitespaceAfter functions to handle cases where the node is null or contains whitespace at the start or end of the text content.

Node insertion logic:

• Updated the useVariablePlugin function to insert whitespace before and after a VariableNode if necessary, ensuring proper spacing.

Mutation listener:

• Modified the useEffect hook to register a mutation listener that cleans up whitespace when a VariableNode is detached.

Additional imports:

• Added the $getNodeByKey import from lexical to support the new mutation listener logic.

[notion-workspace]

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