New json_nextract() function that works like json_extract() except that it

New json_nextract() function that works like json_extract() except that it
returns NULL instead of raising an error if the first argument is not
well-formed JSON.  Or if the first argument is not well-formed JSON and
the second argument is '$', then return the first argument quoted.  The
"->" and "->>" operators are converted to use json_nextract().

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