fix(parser): allow negative numbers

made adding a minus sign before a group/primary expression implicitly multiply by -1

src/math/parser.rs

@@ -1,3 +1,5 @@
+use rust_decimal::dec;
+
 use crate::math::{
     lexer::Token,
     types::{Expression, Operator},
@@ -126,6 +128,15 @@ impl Parser {
                 self.position += 1;
                 Expression::Number(num)
             }
+            Token::Operator(Operator::Subtract) => {
+                self.position += 1;
+                let right = self.parse_group();
+                Expression::Operation(
+                    Operator::Multiply,
+                    Box::new(Expression::Number(dec!(-1))),
+                    Box::new(right),
+                )
+            }
             Token::Identifier(identifier) => {
                 let is_function =
                     matches!(self.tokens.get(self.position + 1), Some(Token::OpenParen));