Add type_of function.

tests/types.rs

@@ -0,0 +1,17 @@
+use rhai::{Engine, EvalAltResult};
+
+#[test]
+fn test_type_of() -> Result<(), EvalAltResult> {
+    let mut engine = Engine::new();
+
+    assert_eq!(engine.eval::<String>("type_of(60 + 5)")?, "i64");
+    assert_eq!(engine.eval::<String>("type_of(1.0 + 2.0)")?, "f64");
+    assert_eq!(
+        engine.eval::<String>(r#"type_of([1.0, 2, "hello"])"#)?,
+        "array"
+    );
+    assert_eq!(engine.eval::<String>(r#"type_of("hello")"#)?, "string");
+    assert_eq!(engine.eval::<String>("let x = 123; x.type_of()")?, "i64");
+
+    Ok(())
+}