Allow usage of functions in predicate expressions

We should extend the predicate function grammar to allow application of functions and not just : : style expressions.

This will allow the predicate expressions to capture more complex use-cases.
```javascript
// Custom type validation (check if email is valid)
isValid(node.email): true
```

Possible syntax that works well when represented in a JSON DSL:

```
apply:
functionName: isValid
arguments: [node.email]
```