fix: ignore underscore in input

I accidentally removed this before... oops :3

src/formatting.rs

@@ -4,6 +4,7 @@ use crate::nodes::{self, Error};
 
 pub fn pretty_eval(input: &str, max_digits: usize) -> Result<String, Error> {
     let (prefix, deliminator, input) = input.rsplit_once_last(&["= ", "& ", "plus.minus "]);
+    let input = input.replace('_', "");
 
     // no need to remove the character here,
     // as we ignore it in all other logic already
@@ -134,6 +135,11 @@ mod tests {
         assert_pretty_eval!("plus.minus 5 & = 5");
     }
 
+    #[test]
+    fn underscore_input() {
+        assert_pretty_eval!("10_000 * 10_000 * 4", "4 * 10^8")
+    }
+
     #[test]
     fn basic_get_significant_digits() {
         let many_zeros = eval("14.00643 + 1.00784 * 4 + 35.446 + 15.9994 * 4");