Improve Variable Naming for Clarity: uniqueDigitsCount & totalCells

[teogor]

Changes:

1. digits to uniqueDigitsCount:

   • Renames the variable digits to uniqueDigitsCount.
   • This change emphasizes the distinct nature of the digits used in the Sudoku grid.
   • The original digits variable remains deprecated with a clear message suggesting the use of uniqueDigitsCount.

2. cells to totalCells:

   • Renames the variable cells to totalCells.
   • This change clarifies the purpose of the variable, referring to the total number of individual squares or cells within the grid.
   • The original cells variable remains deprecated with a message promoting the use of totalCells.