ghc-experimental: make JSVal abstract in GHC.Wasm.Prim

This commit makes JSVal an abstract type in the export list of
GHC.Wasm.Prim. JSVal's internal representation is supposed to be a non
user facing implementation detail subject to change at any time. We
should only expose things that are newtypes of JSVal, not JSVal
itself.

(cherry picked from commit 8037f487)