Added Ligatures to the terminal

[fayaz-modz]

Added ligatures to the terminal. To enable the ligatures simply do:

xterm.TerminalView(
                      terminal,
                      terminalController,
                      textStyle: xterm.TerminalStyle(
                        fontFamily: 'JetBrains Mono',  // any supported font
                        ligatures: true, // enable ligatures
                      ),
);