feat: add keep_highest method to Outcome

01.workspace/oxidice_lib/examples/roll_4d6_keep_3.rs

@@ -0,0 +1,11 @@
+use oxidice_lib::dice::*;
+
+fn main() {
+    let roll = Handful::grab(4, 6).roll();
+    print!("{roll} -> ");
+    let roll = roll.keep_highest(3).unwrap();
+    print!("{roll} -> ");
+    let roll = roll.sum().unwrap();
+    print!("{roll}");
+    println!();
+}