Ignore trailing commas as a builder parameter

Following up from this closed request.

GsonBuilder should have a new property ignoreTrailingCommas().

Most of the JavaScript[0] ecosystem, and a lot of new languages have adopted the usecase of ignoring trailing commas. While not part of the true JSON specification it would be very convenient to move GSON forward with the rest of the ecosystem.

[0] https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Trailing_commas