LibJS: Add BigInt

Author: https://github.com/linusg
Commit: 0ff9d7e189
Pull-request: https://github.com/SerenityOS/serenity/pull/2509
Reviewed-by: https://github.com/alimpfard

Libraries/LibJS/Tests/object-basic.js

@@ -15,6 +15,7 @@ try {
         duplicate: "world"
     };
     assert(o[1] === 23);
+    assert(o[1n] === 23);
     assert(o["1"] === 23);
     assert(o.foo === "bar");
     assert(o["foo"] === "bar");
@@ -30,6 +31,9 @@ try {
     o[10] = "123";
     assert(o[10] === "123");
     assert(o["10"] === "123");
+    o[10n] = "123";
+    assert(o[10] === "123");
+    assert(o["10"] === "123");
     o[-1] = "hello friends";
     assert(o[-1] === "hello friends");
     assert(o["-1"] === "hello friends");