[bigint] Implement BigInt type and basic arithmetic operations

[forfudan]

This pull request introduces new modules for BigInt arithmetic operations and string manipulation in the DeciMojo library and includes significant updates to the README.md file. The most important changes include enhancements to the documentation, implementation of new arithmetic functions for BigInt, and string parsing utilities.

Documentation Updates:

• README.md: Updated the description of DeciMojo to reflect its comprehensive nature and added information about the development of BigInt and BigDecimal types. Clarified the nomenclature section and updated the citation title.

New Modules for BigInt Arithmetic:

• src/decimojo/bigint/__init__.mojo: Added a new sub-package for big integer arithmetic with appropriate licensing information.
• src/decimojo/bigint/arithmetics.mojo: Implemented basic arithmetic functions (add, subtract, negative, absolute) for the BigInt type.
• src/decimojo/bigint/comparison.mojo: Implemented comparison functions for BigInt objects, including compare_absolute.

String Manipulation Utilities:

• src/decimojo/str.mojo: Added functions for parsing strings of numbers into normalized parts, handling various characters and formats.

Additional To-Do Items:

• docs/todo.md: Added a new task to implement methods for augmented arithmetic assignments to improve memory efficiency and performance.