BufferCell constructor
      
      BufferCell({ 
    
- required String char,
- AnsiColorType? fg,
- AnsiColorType? bg,
- Set<FontStyle> ? styles,
Creates a BufferCell with the given char, optional fg, bg,
and a set of styles.
If styles is not provided, it defaults to an empty set.
Implementation
BufferCell({required this.char, this.fg, this.bg, Set<FontStyle>? styles})
  : styles = {
      ...(styles ?? {}),
    };