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)
(cherry picked from commit a834b03d)