[Feature] Added comments extraction from referenced excel sheet and sheet visibility

[rejero]

Linked Issues

Fix #232
Fix #241

Key Features:

• Added functionality to retrieve cell comments from an excel file.
• Added unit tests for a sample excel with different comment conditions.
• Added method to obtain a sheet's visibility

Example Usage:

/*
 * To access comments, simply assign a Data? object to a variable and access the comment
 * 
 * @Return: String?
 * @Desc: cell's comment
*/
Data? cell
String? cellComment = cell.comment; // Returns null if no comments are available


/*
 * To access sheet visibility
 * @Return: Map<String, String>
 * @Desc: map of sheet names and their visibility status
*/
Excel excel
excel.getSheetVisibility();

Future Work:

Additional work to be done is to retrieve the comment's text styling as well, instead of a simple String.
More work can be done to test for any breaking conditions.

[rejero]

Resolves #232

[rejero]

Added an additional commit that provides the visibility of sheets via excel.getSheetVisibility()
Resolves #241

[rejero]

@justkawal Fixed merge conflicts

[rejero]

Fixed merge conflicts. Please review and decide if to merge with main