Introduced DequeBase<T> (abstract base class) to follow SOLID principles and define the contract for double-ended queues.

Added Deque<T> implementation using List.empty(growable: true) internally.

• appendRight(T value)
• appendLeft(T value)
• popRight()
• popLeft()
• peekRight()
• peekLeft()
• isEmpty, length, size
• clear()
• prettyPrint() for horizontal array-like output.

Added unit tests (deque_test.dart) covering:

• Empty deque behavior
• Append and Pop operations from both ends
• Mixed usage scenarios
• Clearing the deque
• Pretty print and toString() output validation

• push, pop, peek, isEmpty, length
• printHeap() for pretty-printing heap levels