Add Tau constant (#2673)

Adds Tau constant using meval::Context.

Also adds a test to match pi's.

Note: Tau ends up not being more precise than 2*pi.

Resolves: #2258

crates/nu-cli/tests/commands/math/eval.rs

@@ -71,3 +71,15 @@ fn evaluates_pi() {
 
     assert!(actual.out.contains("3.14"));
 }
+
+#[test]
+fn evaluates_tau() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+        math eval tau
+        "#
+    ));
+
+    assert!(actual.out.contains("6.28"));
+}