feat(utils): string utility module

[olumo-oke]

** Description
This pull request introduces a new string utility module, string.tact, designed for use in Tact smart contracts on the TON blockchain. This module provides essential string manipulation functionalities, making it easier for developers to handle string operations within their smart contracts.

** Features
The string.tact includes the following string operations:

• String Concatenation: Combine two strings into one.
• Substring Extraction: Extract a substring from a given string.
• String Length: Determine the length of a string.
• String Comparison: Compare two strings for equality.
• String Conversion: Convert integers to strings and vice versa.
• Character Access: Access individual characters in a string.
• Unicode Support: Handle strings with Unicode characters.
• Regular Expressions: Basic pattern matching with regular expressions.
• Trimming and Padding: Trim whitespace from strings and pad strings to a specific length.

** Notes

• Ensure that the Tact environment is set up correctly in your development environment.
• The module is designed to be reusable across multiple smart contracts, promoting code reuse and maintainability.

** Conclusion
This pull request adds a powerful and versatile string utility module to our Ton-ion smart contract library. By integrating this module, we can streamline string operations in our smart contracts, improving development efficiency and code quality.