Converts the supplied string to a JSONArray which can be
interrogated further.
If the input string doesn't have the syntax of a JSON array,
a zero-length JSONArray will be returned.
The JSON parsing is currently somewhat lenient, for instance
allowing a comma before a closing square bracket.
Parameters:
txt(String)
string assumed to contain a JSON array
Return Value (JSONArray):
JSON array value on which further methods can be called