LibJS: Add symbol objects

This commit adds the following classes: SymbolObject, SymbolConstructor,
SymbolPrototype, and Symbol. This commit does not introduce any
new functionality to the Object class, so they cannot be used as
property keys in objects.

Libraries/LibJS/Tests/Symbol.keyFor.js

@@ -0,0 +1,31 @@
+load("test-common.js")
+
+try {
+    const localSym = Symbol("foo");
+    const globalSym = Symbol.for("foo");
+
+    assert(Symbol.keyFor(localSym) === undefined);
+    assert(Symbol.keyFor(globalSym) === "foo");
+
+    const testThrows = (value, str) => {
+        assertThrowsError(() => {
+            Symbol.keyFor(value);
+        }, {
+            error: TypeError,
+            message: str + " is not a symbol",
+        });
+    };
+
+    testThrows(1, "1");
+    testThrows(null, "null");
+    testThrows(undefined, "undefined");
+    testThrows([], "[object Array]");
+    testThrows({}, "[object Object]");
+    testThrows(true, "true");
+    testThrows("foobar", "foobar");
+    testThrows(function(){}, "[object ScriptFunction]"); // FIXME: Better function stringification
+
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}