Add double equal to functionality

Add double = functionality. But be careful as to how to implement it when bodmas is present. Ideal for 2 number calculator only.

Look at the existing calculator logic in the codebase, likely in a main Python file handling arithmetic operations. Understand how single '=' is processed and where BODMAS rules are applied. Add a '==' operator that triggers calculation on the current expression. Test with simple two-number inputs to ensure it works before and after BODMAS operations.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.