feat: add constant pi

copy and pasted from ALEKS calculator,
as I gave up trying to find what the best approximation to use would be.

src/nodes/from_str.rs

@@ -9,6 +9,7 @@ impl Node {
         Ok(match s {
             "c" => Self::Number(Float::with_val(PREC, 299_792_458)),
             "A" => Self::Number(Float::with_val(PREC, 6.02214076e23)),
+            "pi" => Self::Number(Float::with_val(PREC, 3.1415926536)),
             _ => Self::Function(s.parse()?),
         })
     }

src/nodes/tests.rs

@@ -22,6 +22,10 @@ fn constants() {
         Nodes::from_str("A").unwrap().evaluate().to_f64(),
         6.02214076e23
     );
+    assert_eq!(
+        Nodes::from_str("pi").unwrap().evaluate().to_f64(),
+        3.1415926536
+    );
 }
 
 #[test]