Update BigInt.json

[gunnarbittersmann]

Firefox may have the javascript.options.bigint flag since version 65, but BigInt doesn’t work in versions 65 and 66.
Test: https://bittersmann.de/test/2019fact.html

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[ddbeck]

Good catch, @gunnarbittersmann! It looks like before version 67, a compile flag was needed in addition to the bigint option: https://bugzilla.mozilla.org/show_bug.cgi?id=1522436

Thank you again and welcome to BCD!