Single Cell Selection

[GimanthaDissanayake]

Implemented the functionality of single-cell selection.

• Created methods in sheet.ts to retrieve column and row indexes
• Added onCellClick attribute to LightSheetOptions in main.types.ts
• Implemented the functionality with some styling in render.ts

Please review and merge. Thank you.

[rafinutshaw]

Some of the styling work is already done in the mainbranch. Kindly pull from and resolve conflicts then I can do a review.

[lakosd]

These changes will be included in a different PR and a different branch