feat: add comprehensive weight unit conversion module

Author: AliAlimohammadi

DIRECTORY.md

@@ -91,6 +91,7 @@
     * [Speed](https://github.com/TheAlgorithms/Rust/blob/master/src/conversions/speed.rs)
     * [Time Units](https://github.com/TheAlgorithms/Rust/blob/master/src/conversions/time_units.rs)
     * [Temperature](https://github.com/TheAlgorithms/Rust/blob/master/src/conversions/temperature.rs)
+    * [Weight](https://github.com/TheAlgorithms/Rust/blob/master/src/conversions/weight.rs)
   * Data Structures
     * [AVL Tree](https://github.com/TheAlgorithms/Rust/blob/master/src/data_structures/avl_tree.rs)
     * [B-Tree](https://github.com/TheAlgorithms/Rust/blob/master/src/data_structures/b_tree.rs)

src/conversions/mod.rs

@@ -20,6 +20,7 @@ mod roman_numerals;
 mod speed;
 mod temperature;
 mod time_units;
+mod weight;
 
 pub use self::binary_to_decimal::binary_to_decimal;
 pub use self::binary_to_hexadecimal::binary_to_hexadecimal;
@@ -45,3 +46,4 @@ pub use self::roman_numerals::{int_to_roman, roman_to_int};
 pub use self::speed::{convert_speed, SpeedUnit};
 pub use self::temperature::{convert_temperature, TemperatureUnit};
 pub use self::time_units::convert_time;
+pub use self::weight::{convert_weight, WeightUnit};