Parse anonymous function with no clauses

[jonatanklosko]

Closes #77.

### Ref 8. Empty anonymous function

As opposed to the Elixir parser, we successfully parse anonymous functions with
no stab clauses, so this is valid:

```
x = fn

end
```

This code may appear if an editor extension automatically inserts `end` after
`fn`. We want it to parse as anonymous function node, so that functionality such
as indentation works as expected.

If we require at least one stab clause, the above would be parsed with an error,
where `fn` and `end` are both identifiers. That is not useful. Note that both
`fn` and `end` are reserved keywords, so there is no case where they would
actually be identifiers, hence no ambiguity.

Ideally, this would parse it as an anonymous function node with an error, however
that does not seem straightforward to achieve.