[decimal] Implement multiply() for BigDecimal type

[forfudan]

This pull request includes several changes to enhance the BigDecimal and BigInt functionalities, improve the arithmetic operations, and update the documentation. The main changes include the addition of the multiply function for BigDecimal, improvements to the add and subtract functions, and updates to the string representation methods.

Enhancements to BigDecimal:

• Added the multiply function to perform multiplication of BigDecimal numbers. [1] [2]
• Improved the add and subtract functions to handle zero operands more efficiently and ensure precise results. [1] [2] [3] [4]
• Added the extend_precision method to increase the precision of BigDecimal numbers by adding trailing zeros.
• Updated the to_string method to support line width limits for better readability. [1] [2]
• Added the internal_representation method to print the internal representation details of BigDecimal numbers.

Enhancements to BigInt:

• Updated the to_string method to support line width limits for better readability. [1] [2]
• Removed unnecessary constants MAX_OF_WORD and BASE_OF_WORD and replaced their usage with direct values. [1] [2]

Documentation updates:

• Updated the README.md to reflect the current state of the BigDecimal implementation and added a table summarizing the core types and their internal representations.

Benchmark updates:

• Added the bench_bigdecimal_multiply benchmark to the benches/bigdecimal/bench.mojo file to include multiplication benchmarks. [1] [2] [3]