feat(lucas_number): add new rust exercise

subjects/lucas_number/README.md

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+## lucas_number
+
+### Instructions
+
+Complete the body of the **function** `lucas_number`.
+
+```rust
+pub fn lucas_number(n: u32) -> u32 {}
+```
+
+This function receives a number `n` and returns the `n`th number in the Lucas Numbers. 
+
+The Lucas Numbers start like this: 2, 1, 3, 4, 7, 11, 18, 29, 47, 76, 123, etc... 
+
+### Usage
+
+Here is a possible test for your function:
+
+```rust
+use lucas_number::lucas_number;
+
+fn main() {
+    println!("The element in the position {} in Lucas Numbres is {}", 2, lucas_number(2));
+    println!("The element in the position {} in Lucas Numbres is {}", 5, lucas_number(5));
+    println!("The element in the position {} in Lucas Numbres is {}", 10, lucas_number(10));
+    println!("The element in the position {} in Lucas Numbres is {}", 13, lucas_number(13));
+}
+```
+
+And its output:
+
+```console
+$ cargo run
+The element in the position 2 in Lucas Numbers is 3
+The element in the position 5 in Lucas Numbers is 11
+The element in the position 10 in Lucas Numbers is 123
+The element in the position 13 in Lucas Numbers is 521
+$
+```