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-preventExtensions.js

@@ -0,0 +1,55 @@
+load("test-common.js");
+
+try {
+    let p = new Proxy({}, { preventExtensions: null });
+    assert(Object.preventExtensions(p) === p);
+    p = new Proxy({}, { preventExtensions: undefined });
+    assert(Object.preventExtensions(p) === p);
+    p = new Proxy({}, {});
+    assert(Object.preventExtensions(p) == p);
+
+    let o = {};
+    p = new Proxy(o, {
+        preventExtensions(target) {
+            assert(target === o);
+            return true;
+        }
+    });
+
+    Object.preventExtensions(o);
+    Object.preventExtensions(p);
+
+    // Invariants
+
+    p = new Proxy({}, {
+        preventExtensions() {
+            return false;
+        },
+    });
+    assertThrowsError(() => {
+        Object.preventExtensions(p);
+    }, {
+        error: TypeError,
+        message: "Proxy preventExtensions handler returned false",
+    });
+
+    o = {};
+    p = new Proxy(o, {
+        preventExtensions() {
+            return true;
+        },
+    });
+    assertThrowsError(() => {
+        Object.preventExtensions(p);
+    }, {
+        error: TypeError,
+        message: "Proxy handler's preventExtensions trap violates invariant: cannot return true if the target object is extensible"
+    });
+
+    Object.preventExtensions(o);
+    assert(Object.preventExtensions(p) === p);
+
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}