[decimal] Implement a subtraction function for BigDecimal (#67)

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]](diffhunk://#diff-96175f1000f81fe074f70a9adc4b39cd3f2dac8e85d30aa7efed9fc02b46e5ecR2)
[[2]](diffhunk://#diff-96175f1000f81fe074f70a9adc4b39cd3f2dac8e85d30aa7efed9fc02b46e5ecR12)
[[3]](diffhunk://#diff-96175f1000f81fe074f70a9adc4b39cd3f2dac8e85d30aa7efed9fc02b46e5ecR21-R25)
* 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.

Author: forfudan

benches/bigdecimal/bench.mojo

@@ -1,4 +1,5 @@
 from bench_bigdecimal_add import main as bench_add
+from bench_bigdecimal_subtract import main as bench_sub
 
 
 fn main() raises:
@@ -8,6 +9,7 @@ fn main() raises:
 This is the BigInt Benchmarks
 =========================================
 add:         Add
+sub:         Subtract
 all:         Run all benchmarks
 q:           Exit
 =========================================
@@ -16,8 +18,11 @@ q:           Exit
     var command = input("Type name of bench you want to run: ")
     if command == "add":
         bench_add()
+    elif command == "sub":
+        bench_sub()
     elif command == "all":
         bench_add()
+        bench_sub()
     elif command == "q":
         return
     else: