[decimal] Implement a subtraction function for BigDecimal

[forfudan]

This pull request includes several changes to the benches/bigdecimal/bench.mojo, mojoproject.toml, and src/decimojo/bigdecimal/arithmetics.mojo files to add new functionality and improve the codebase. The most important changes include adding a new subtraction benchmark, renaming benchmark commands, and implementing a subtraction function for BigDecimal.

New functionality:

• Added a new subtraction benchmark by importing bench_bigdecimal_subtract and updating the benchmark commands in benches/bigdecimal/bench.mojo. [1] [2] [3]
• Implemented the subtract function for BigDecimal in src/decimojo/bigdecimal/arithmetics.mojo, including handling zero operands and ensuring operands have the same scale.

Codebase improvements:

• Renamed benchmark commands in mojoproject.toml for consistency and clarity.
• Added notes to the add function in src/decimojo/bigdecimal/arithmetics.mojo to clarify its behavior and result properties.
• Added the __sub__ method to the BigDecimal struct in src/decimojo/bigdecimal/bigdecimal.mojo to support subtraction using the new subtract function.