Add undo system with coalescing logic and comprehensive tests.

- Implemented robust undo system supporting coalescing of text operations (insert, backspace, delete).
- Added `UndoSystem` integration into the editor/commands pipeline.
- Wrote extensive unit tests for various undo/redo scenarios, including multiline operations, cursor preservation, and history management.
- Refactored to ensure consistent cursor behavior during undo/redo actions.
- Updated CMake to include new tests.

UndoSystem.h

@@ -33,6 +33,11 @@ public:
 	void UpdateBufferReference(Buffer &new_buf);
 
 private:
+	enum class PendingAppendMode : std::uint8_t {
+		Append,
+		Prepend,
+	};
+
 	void apply(const UndoNode *node, int direction); // +1 redo, -1 undo
 	void free_node(UndoNode *node);
 
@@ -48,6 +53,8 @@ private:
 
 	void update_dirty_flag();
 
+	PendingAppendMode pending_mode_ = PendingAppendMode::Append;
+
 	Buffer *buf_;
 	UndoTree &tree_;
 };