[bigint] Add to_int and improve from_int

[forfudan]

This pull request includes several changes to the BigInt structure in the src/decimojo/bigint/bigint.mojo file to improve its functionality and add new initialization methods. Additionally, new tests have been added to validate these changes.

Enhancements to BigInt structure:

• src/decimojo/bigint/bigint.mojo: Added new initialization methods for BigInt including __init__ with different parameter signatures and the to_int method for converting BigInt to Int. Updated the from_components method to from_raw_words and improved documentation and examples.

New tests:

• tests/bigint/test_bigint_to_int.mojo: Added new tests to validate the to_int method and __int__ operator for various numerical cases, including edge cases and error scenarios.