LibJS: Throw TypeError on write to non-writable property in strict mode

Author: https://github.com/IdanHo
Commit: https://github.com/SerenityOS/serenity/commit/2a8f4f097c3
Pull-request: https://github.com/SerenityOS/serenity/pull/7825
Reviewed-by: https://github.com/linusg

Userland/Libraries/LibJS/Tests/non-writable-assignment.js

@@ -0,0 +1,12 @@
+test("normal mode", () => {
+    expect(() => {
+        NaN = 5; // NaN is a non-writable global variable
+    }).not.toThrow();
+});
+
+test("strict mode", () => {
+    expect(() => {
+        "use strict";
+        NaN = 5; // NaN is a non-writable global variable
+    }).toThrowWithMessage(TypeError, "Cannot write to non-writable property 'NaN'");
+});