LibJS: Support @@toPrimitive in ToPrimitive abstract operation

Fixes #3961.

Userland/Libraries/LibJS/Tests/custom-@@toPrimitive.js

@@ -0,0 +1,20 @@
+test("basic functionality", () => {
+    const o = {
+        [Symbol.toPrimitive]: hint => {
+            lastHint = hint;
+        },
+    };
+    let lastHint;
+
+    // Calls ToPrimitive abstract operation with 'string' hint
+    String(o);
+    expect(lastHint).toBe("string");
+
+    // Calls ToPrimitive abstract operation with 'number' hint
+    +o;
+    expect(lastHint).toBe("number");
+
+    // Calls ToPrimitive abstract operation with 'default' hint
+    "" + o;
+    expect(lastHint).toBe("default");
+});