This option skips the usual decoding step and instead prints out a JSON-
formatted list of the variables that are referenced by the configuration.
In simple cases this is not required, but for more complex use-cases it
can be useful to first analyze the input to see which variables need to
be in the scope, then construct a suitable set of variables before finally
decoding the input. For example, some of the variable values may be
expensive to produce.
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