[test] Add separate testing files for __int__, __float__, __str__

[forfudan]

This pull request includes several changes to the DeciMojo library, focusing on code cleanup, new test additions, and minor optimizations. The most important changes include the addition of a new test case, a fix in the Decimal struct, and the removal of unnecessary comment lines.

New test additions:

• mojoproject.toml: Added a new test case test_to_float to the test suite.

Test removal:

• tests/test_conversions.mojo: Removed the entire file containing tests for Decimal conversion methods and split it into test_to_string and test_to_int.

Code optimizations:

• src/decimojo/decimal.mojo: Fixed the to_float method in the Decimal struct to correctly use Float64(10) for division, ensuring proper floating-point arithmetic.

Documentation updates:

• docs/todo.md: Marked the optimization task for the exp() function as completed.