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/CMakeLists.txt

@@ -46,6 +46,10 @@ set(SOURCES
     Runtime/StringConstructor.cpp
     Runtime/StringObject.cpp
     Runtime/StringPrototype.cpp
+    Runtime/Symbol.cpp
+    Runtime/SymbolConstructor.cpp
+    Runtime/SymbolObject.cpp
+    Runtime/SymbolPrototype.cpp
     Runtime/Uint8ClampedArray.cpp
     Runtime/Value.cpp
     Token.cpp