Allow std.json to overlook trailing comma

The JSON grammar does not allow trailing commas, however they are accepted by many JSON parsers.
$(MREF std,json) now ignores trailing commas as well. Use $(REF_SHORT JSONOptions.strictParsing, std,json) to disable this behavior.

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import std.json;
import std.exception : assertThrown, assertNotThrown;

// before
assertThrown(parseJSON(`{ "a" : { } , }`));

// after
assertNotThrown(parseJSON(`{ "a" : { } , }`));
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