LibJS: Add spreading in object literals

Supports spreading strings, arrays, and other objects within object
literals.

Libraries/LibJS/Tests/object-spread.js

@@ -0,0 +1,72 @@
+load("test-common.js");
+
+function testObjSpread(obj) {
+    return obj.foo === 0 && 
+        obj.bar === 1 &&
+        obj.baz === 2 &&
+        obj.qux === 3;
+}
+
+function testObjStrSpread(obj) {
+    return obj[0] === "a" && 
+        obj[1] === "b" && 
+        obj[2] === "c" &&
+        obj[3] === "d";
+}
+
+try {
+    let obj = { 
+        foo: 0, 
+        ...{ bar: 1, baz: 2 }, 
+        qux: 3,
+    };
+    assert(testObjSpread(obj));
+
+    obj = { foo: 0, bar: 1, baz: 2 };
+    obj.qux = 3;
+    assert(testObjSpread({ ...obj }));
+
+    let a = { bar: 1, baz: 2 };
+    obj = { foo: 0, ...a, qux: 3 };
+    assert(testObjSpread(obj));
+
+    obj = {
+        ...{},
+        ...{
+            ...{ foo: 0, bar: 1, baz: 2 },
+        },
+        qux: 3,
+    };
+    assert(testObjSpread(obj));
+
+    obj = { ..."abcd" };
+    assert(testObjStrSpread(obj));
+    
+    obj = { ...["a", "b", "c", "d"] };
+    assert(testObjStrSpread(obj));
+    
+    obj = { ...String("abcd") };
+    assert(testObjStrSpread(obj));
+
+    a = { foo: 0 };
+    Object.defineProperty(a, 'bar', {
+        value: 1,
+        enumerable: false,
+    });
+    obj = { ...a };
+    assert(obj.foo === 0 && obj.bar === undefined);
+
+    let empty = ({
+        ...undefined,
+        ...null,
+        ...1,
+        ...true,
+        ...function(){},
+        ...Date,
+    });
+    assert(Object.getOwnPropertyNames(empty).length === 0);
+
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}