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-get.js

@@ -0,0 +1,71 @@
+load("test-common.js");
+
+try {
+    assert((new Proxy({}, { get: undefined })).foo === undefined);
+    assert((new Proxy({}, { get: null })).foo === undefined);
+    assert((new Proxy({}, {})).foo === undefined);
+
+    let o = {};
+    let p = new Proxy(o, {
+        get(target, property, receiver) {
+            assert(target === o);
+            assert(property === "foo");
+            assert(receiver === p);
+        },
+    });
+
+    p.foo;
+
+    o = { foo: 1 };
+    p = new Proxy(o, {
+        get(target, property, receiver) {
+            if (property === "bar") {
+                return 2;
+            } else if (property === "baz") {
+                return receiver.qux;
+            } else if (property === "qux") {
+                return 3;
+            }
+            return target[property];
+        }
+    });
+
+    assert(p.foo === 1);
+    assert(p.bar === 2);
+    assert(p.baz === 3);
+    assert(p.qux === 3);
+    assert(p.test === undefined);
+
+    // Invariants
+
+    o = {};
+    Object.defineProperty(o, "foo", { value: 5, configurable: false, writable: true });
+    Object.defineProperty(o, "bar", { value: 10, configurable: false, writable: false });
+
+    p = new Proxy(o, {
+        get() {
+            return 8;
+        },
+    });
+
+    assert(p.foo === 8);
+
+    assertThrowsError(() => {
+        p.bar;
+    }, {
+        error: TypeError,
+        message: "Proxy handler's get trap violates invariant: the returned value must match the value on the target if the property exists on the target as a non-writable, non-configurable own data property",
+    });
+
+    Object.defineProperty(o, "baz", { configurable: false, set(_) {} });
+    assertThrowsError(() => {
+        p.baz;
+    }, {
+        error: TypeError,
+        message: "Proxy handler's get trap violates invariant: the returned value must be undefined if the property exists on the target as a non-configurable accessor property with an undefined get attribute",
+    });
+
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}