Add `fs.typeof` method

It returns the path type according to its on-disk representation.

Possible results include: `file`, `directory`, `symlink`, `socket`, `fifo`, `characterDevice`, `blockDevice`

```ts
// The path argument may be absolute or relative to the root directory.
fs.typeof('foo')
```

The best use case for this is a `switch` statement.

```ts
switch (fs.typeof(path)) {
case 'file':
// ...
break

case 'directory':
// ...
break

default:
throw new Error('unsupported file type')
}
```

This is better than using `fs.stat` or `fs.lstat`, because `change` events don't need to be watched for like they do with those methods. Just need to use the `existenceWatcher`, I think.