[dict] retrieve function

[ghostsquad]

currently get looks like this:

get $myDict "key1"

which supports a workflow such as coalesce "foo" "" | get $myDict

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This is a request to support the opposite workflow:

retrieve "key1" $myDict

which could be used like:

`. | retrieve  "value"`

I'm not really sure if retrieve if the right verb here, but order matters if you want succinct code and to use pipes in go templates.