add @ operator to blues jsonata

[tbal999]

Firstly - excellent work porting jsonata to Golang. Impressive. I have a request:

In original JSONATA they have # and @ operators which enable JOIN like operations on data (a simplification on top of usage of map and filter) i.e

$$.new.additionalfeatures @ $T1.$$.old.items @ $T2[$T1.id = $T2.id].{
    "id": $T1.id,
    "color": $T1.colour,
    "name": $T2.name
}

and currently you'd need to do something like

$$.old.items.{
    "id": $.id,
    "name": $.name,
    "color": ($filter($$.new.additionalfeatures, function($i){$i.id = $.id})).colour
}

It would be useful to have at least the @ operator added to the Go port of jsonata.

I appreciate this would be a fiddly bit of work - so it's a 'nice to have'.

[tbal999]

In the original jsonata - here is the PR that introduced @ and # operators to the codebase: https://github.com/jsonata-js/jsonata/pull/371/files#diff-54c345dc104dc19440f9c2482b7883df820e8b9b699fdd8fa07e2773e7197a29