fix: obey arrowParens=always on singular inferred lambda parameters

Author: jtkiesel

src/printers/expressions.ts

@@ -682,6 +682,11 @@ function printLambdaExpressionSignature(
     } else {
       parts.push(parameters);
     }
+
+    if (path.node.parametersNode.type === SyntaxType.Identifier) {
+      parts.unshift("(");
+      parts.push(")");
+    }
   }
 
   const dangling = printDanglingComments(path);

test/unit-test/lambda/arrow-parens-always/.prettierrc.json

@@ -0,0 +1,3 @@
+{
+  "arrowParens": "always"
+}