LibJS: Add Proxy objects

Includes all traps except the following: [[Call]], [[Construct]],
[[OwnPropertyKeys]].

An important implication of this commit is that any call to any virtual
Object method has the potential to throw an exception. These methods
were not checked in this commit -- a future commit will have to protect
these various method calls throughout the codebase.

Libraries/LibJS/Tests/Proxy.handler-deleteProperty.js

@@ -0,0 +1,56 @@
+load("test-common.js");
+
+try {
+    assert(delete (new Proxy({}, { deleteProperty: undefined })).foo === true);
+    assert(delete (new Proxy({}, { deleteProperty: null })).foo === true);
+    assert(delete (new Proxy({}, {})).foo === true);
+
+    let o = {};
+    let p = new Proxy(o, {
+        deleteProperty(target, property) {
+            assert(target === o);
+            assert(property === "foo");
+            return true;
+        }
+    });
+
+    delete p.foo;
+
+    o = { foo: 1, bar: 2 };
+    p = new Proxy(o, {
+        deleteProperty(target, property) {
+            if (property === "foo") {
+                delete target[property];
+                return true;
+            }
+            return false;
+        }
+    });
+
+    assert(delete p.foo === true);
+    assert(delete p.bar === false);
+
+    assert(o.foo === undefined);
+    assert(o.bar === 2);
+
+    // Invariants
+
+    o = {};
+    Object.defineProperty(o, "foo", { configurable: false });
+    p = new Proxy(o, {
+        deleteProperty() {
+            return true;
+        },
+    });
+
+    assertThrowsError(() => {
+        delete p.foo;
+    }, {
+        error: TypeError,
+        message: "Proxy handler's delete trap violates invariant: cannot report a non-configurable own property of the target as deleted",
+    });
+
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}