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.js

@@ -0,0 +1,26 @@
+load("test-common.js")
+
+try {
+    const s1 = Symbol("foo");
+    const s2 = Symbol("foo");
+
+    assert(s1 !== s2);
+    assert(s1.description === "foo");
+    assert(s2.description === "foo");
+
+    s1.description = "bar";
+    assert(s1.description === "foo");
+
+    assert(typeof s1 === "symbol");
+
+    assertThrowsError(() => {
+        Symbol(Symbol('foo'));
+    }, {
+        error: TypeError,
+        message: "Can't convert symbol to string"
+    })
+    
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}