[bigint] Implement multiply for BigInt

[forfudan]

This pull request focuses on enhancing the BigInt type in the decimojo library by adding multiplication functionality and comprehensive tests. The most important changes include the addition of the multiply function, modifications to the BigInt structure to support the new functionality, and the creation of extensive tests to ensure the correctness of the multiplication operation.

Enhancements to BigInt functionality:

• Added the multiply function to perform multiplication of two BigInt numbers in src/decimojo/bigint/arithmetics.mojo.
• Introduced the __mul__ and __imul__ methods to the BigInt structure to support the multiplication operator.

Modifications to BigInt structure:

• Replaced the alias _words_type with List[UInt32] for the words field in the BigInt structure.
• Updated the __init__ methods to use List[UInt32] directly for initializing the words field.
• Added utility methods is_one_or_minus_one and is_negative to the BigInt structure for special case handling.

Comprehensive tests for multiplication:

• Created a new test file tests/bigint/test_bigint_multiply.mojo with extensive test cases covering basic multiplication, special cases, negative numbers, large numbers, and the commutative property.