added mergeSort implementation in dart
added mergeSort implementation in dart

ZoranPandovski/al-go-rithms,ZoranPandovski/al-go-rithms,ZoranPandovski/al-go-rithms,ZoranPandovski/al-go-rithms,ZoranPandovski/al-go-rithms,ZoranPandovski/al-go-rithms,ZoranPandovski/al-go-rithms,ZoranPandovski/al-go-rithms,ZoranPandovski/al-go-rithms,ZoranPandovski/al-go-rithms,ZoranPandovski/al-go-rithms,ZoranPandovski/al-go-rithms,ZoranPandovski/al-go-rithms,ZoranPandovski/al-go-rithms,ZoranPandovski/al-go-rithms,ZoranPandovski/al-go-rithms,ZoranPandovski/al-go-rithms,ZoranPandovski/al-go-rithms,ZoranPandovski/al-go-rithms,ZoranPandovski/al-go-rithms