LibJS: Start implementing Date :^)

This adds:

- A global Date object (with `length` property and `now` function)
- The Date constructor (no arguments yet)
- The Date prototype (with `get*` functions)

Libraries/LibJS/Runtime/Value.cpp

@@ -58,7 +58,7 @@ String Value::to_string() const
             return "NaN";
 
         // FIXME: This needs improvement.
-        return String::number((i32)as_double());
+        return String::format("%f", as_double());
     }
 
     if (is_object())