LibJS: Add JSON.stringify

Author: https://github.com/mattco98
Commit: 39576b2238
Pull-request: https://github.com/SerenityOS/serenity/pull/2545
Reviewed-by: https://github.com/Dexesttp
Reviewed-by: https://github.com/awesomekling

Libraries/LibJS/Tests/JSON.stringify-proxy.js

@@ -0,0 +1,18 @@
+load("test-common.js");
+
+try {
+    let p = new Proxy([], {
+        get(_, key) {
+            if (key === "length")
+                return 3;
+            return Number(key);
+        },
+    });
+
+    assert(JSON.stringify(p) === "[0,1,2]");
+    assert(JSON.stringify([[new Proxy(p, {})]]) === "[[[0,1,2]]]");
+
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}