feat(tvix/eval): implement builtins.typeOf

Change-Id: Ibc5039b444fadf6f9e5cd9132fcd825a871cee06
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6261
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/builtins/mod.rs

@@ -60,6 +60,9 @@ fn pure_builtins() -> Vec<Builtin> {
             // TODO: toString is actually not the same as Display
             Ok(Value::String(format!("{}", args[0]).into()))
         }),
+        Builtin::new("typeOf", 1, |args| {
+            Ok(Value::String(args[0].type_of().into()))
+        }),
     ]
 }