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-has.js

@@ -0,0 +1,62 @@
+load("test-common.js");
+
+try {
+    assert("foo" in new Proxy({}, { has: null }) === false);
+    assert("foo" in new Proxy({}, { has: undefined}) === false);
+    assert("foo" in new Proxy({}, {}) === false);
+
+    let o = {};
+    let p = new Proxy(o, {
+        has(target, prop) {
+            assert(target === o);
+            assert(prop === "foo");
+            return true;
+        }
+    });
+
+    "foo" in p;
+
+    p = new Proxy(o, {
+        has(target, prop) {
+            if (target.checkedFoo)
+                return true;
+            if (prop === "foo")
+                target.checkedFoo = true;
+            return false;
+        }
+    });
+
+    assert("foo" in p === false);
+    assert("foo" in p === true);
+
+    // Invariants
+
+    o = {};
+    Object.defineProperty(o, "foo", { configurable: false });
+    Object.defineProperty(o, "bar", { value: 10, configurable: true });
+    p = new Proxy(o, {
+        has() {
+            return false;
+        }
+    });
+
+    assertThrowsError(() => {
+        "foo" in p;
+    }, {
+        error: TypeError,
+        message: "Proxy handler's has trap violates invariant: a property cannot be reported as non-existent if it exists on the target as a non-configurable property",
+    });
+
+    Object.preventExtensions(o);
+
+    assertThrowsError(() => {
+        "bar" in p;
+    }, {
+        error: TypeError,
+        message: "Proxy handler's has trap violates invariant: a property cannot be reported as non-existent if it exist on the target and the target is non-extensible",
+    });
+
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}