Simplify creating variables in filters

Make the function also a proxy

```diff
export const contains: Filter = ({ subject, object, variable }) => sparql`
- ${subject} ${rdfs.label} ${variable('label')} .
+ ${subject} ${rdfs.label} ${variable.label} .
  
- FILTER( REGEX(${variable('label')}, "${object.value}", "i") )
+ FILTER( REGEX(${variable.label}, "${object.value}", "i") )
`
```