Fix type inferencing and syntax highlighting for boolean category methods

Consider the following:
```groovy
'42'.number
' \t'.allWhitespace
```

Expressions "number" and "allWhitespace" are showing as unknown (underlined).

![image](https://user-images.githubusercontent.com/18193802/73700902-800f6300-46ad-11ea-946a-cd23b83c2106.png)