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.js

@@ -0,0 +1,30 @@
+load("test-common.js");
+
+try {
+    new Proxy({}, {});
+
+    assertThrowsError(() => {
+        new Proxy();
+    }, {
+        error: TypeError,
+        message: "Proxy requires at least two arguments",
+    });
+
+    assertThrowsError(() => {
+        Proxy();
+    }, {
+        error: TypeError,
+        message: "Proxy must be called with the \"new\" operator",
+    });
+
+    assertThrowsError(() => {
+        new Proxy(1, {});
+    }, {
+        error: TypeError,
+        message: "Expected target argument of Proxy constructor to be object, got 1",
+    });
+
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}