[decimal] Make Decimal type trivial + Add to_str_scientific() method

[forfudan]

This pull request includes several changes to the Decimal type:

Code simplification and refactoring:

• src/decimojo/decimal.mojo: Removed redundant comments and reorganized the docstring for the Decimal struct to improve clarity.
• src/decimojo/decimal.mojo: Made Decimal a trivial type. Removed the __copyinit__ method.
• src/decimojo/decimal.mojo: Removed several reverse arithmetic operation dunders (__radd__, __rsub__, __rmul__, __rtruediv__) for Float64 because it is not exact.
• src/decimojo/decimal.mojo: Moved the string representation logic from the __str__ method to a new to_str method and added a to_str_scientific method for scientific notation.

Enhancements to rounding mode:

• src/decimojo/rounding_mode.mojo: Added a docstring to the RoundingMode struct and included notes about the current workaround for defining custom enum-like classes in Mojo.