add tokenizer for better expression evaluation

Author: yinebebt

README.md

@@ -50,4 +50,6 @@ Performs basic mathematical operations.
 - Addition: `+`
 - Subtraction: `-`
 - Multiplication: `*`
-- Division: `/`
+- Division: `/`
+- Parentheses: `()` for grouping
+- Negative numbers: `-5`, `(-3 + 2)`