Export `SimpleSlice` in fragments.js

[TimonVS]

All fragment types are being exported, except for SimpleSlice (https://github.com/prismicio/javascript-kit/blob/master/lib/fragments.js#L1398:L1420).
I need to extend SimpleSlice for a custom parser.

[arnaudlewis]

No problem @TimonVS, it will be available in the next version. It should be released in the next few days.