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-isExtensible.js

@@ -0,0 +1,49 @@
+load("test-common.js");
+
+try {
+    assert(Object.isExtensible(new Proxy({}, { isExtensible: null })) === true);
+    assert(Object.isExtensible(new Proxy({}, { isExtensible: undefined })) === true);
+    assert(Object.isExtensible(new Proxy({}, {})) === true);
+
+    let o = {};
+    let p = new Proxy(o, {
+        isExtensible(target) {
+            assert(target === o);
+            return true;
+        }
+    });
+
+    Object.isExtensible(p);
+
+    // Invariants
+
+    o = {};
+    p = new Proxy(o, {
+        isExtensible(proxyTarget) {
+            assert(proxyTarget === o);
+            return true;
+        },
+    });
+
+    assert(Object.isExtensible(p) === true);
+    Object.preventExtensions(o);
+
+    assertThrowsError(() => {
+        Object.isExtensible(p);
+    }, {
+        error: TypeError,
+        message: "Proxy handler's isExtensible trap violates invariant: return value must match the target's extensibility",
+    });
+
+    p = new Proxy(o, {
+        isExtensible(proxyTarget) {
+            assert(proxyTarget === o);
+            return false;
+        },
+    });
+    assert(Object.isExtensible(p) === false);
+
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}