This starts the transition to a new method of documenting the cfgs that are
enabled for a given crate in the json file. This is changing from a list
of atoms and a dict of key:value pairs, to a list of strings that is
equivalent to that returned by `rustc --print cfg ..`, and parsed in the
same manner by rust-analyzer.
This is the first of two changes, which adds the new field that contains
the list of strings. Next change will complete the transition and remove
the previous fields.
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