[decimal] Implement floor_divide and modulo functions (// and %) (#45)

This pull request introduces several new features and improvements to
the `decimojo` library, focusing on arithmetic operations, benchmarking,
and rounding modes. The most important changes include adding new
arithmetic functions, updating benchmarks, and refining the rounding
mode implementation.

### New Arithmetic Functions:
* Added `floor_divide` and `modulo` functions to the `arithmetics.mojo`
file. These functions provide integral division and remainder operations
for `Decimal` types.

### Benchmark Updates:
* Updated `benches/bench.mojo` to include new benchmarks for
`floor_divide` and `modulo` operations.
* Updated `mojoproject.toml` to include test and benchmark commands for
`floor_divide` and `modulo`.

### Rounding Mode Improvements:
* Refined the `RoundingMode` struct by renaming methods and aliases to
use lowercase names, improving code readability and consistency.

### Code Simplification:
* Simplified the `Decimal` struct by removing unnecessary exception
handling in arithmetic dunder methods.
* Removed redundant scientific exponent calculation method from the
`Decimal` struct.

### Miscellaneous:
* Updated the `__init__.mojo` file to import the new `divide`,
`floor_divide`, and `modulo` functions.
* Fixed the rounding mode in the `Decimal` struct to use the correct
constant names.

Author: forfudan

benches/bench.mojo

@@ -2,6 +2,8 @@ from bench_add import main as bench_add
 from bench_subtract import main as bench_subtract
 from bench_multiply import main as bench_multiply
 from bench_divide import main as bench_divide
+from bench_floor_divide import main as bench_floor_divide
+from bench_modulo import main as bench_modulo
 from bench_sqrt import main as bench_sqrt
 from bench_from_float import main as bench_from_float
 from bench_from_string import main as bench_from_string
@@ -21,6 +23,8 @@ fn main() raises:
     bench_subtract()
     bench_multiply()
     bench_divide()
+    bench_floor_divide()
+    bench_modulo()
     bench_sqrt()
     bench_from_float()
     bench_from_string()