Added static content convention extensions

A couple of extension methods that adds the functionality of `StaticContentConventionsBuilder` onto the static conventions

Helps shorting the syntax by enabling

```c#
conventions.StaticContentsConventions.Add(
   StaticContentConventionBuilder.AddDirectory("assets", @"contentFolder/subFolder")
);
```

to be converted to

```c#
conventions.StaticContentsConventions.AddDirectory("assets", @"contentFolder/subFolder");
```

and the same for `AddFile`