Add details on how Int is represented when compiling to Javascript

[JamieBallingall]

The documentation for Int in Prim says:
"A 32-bit signed integer. See the purescript-integers package for details of how this is accomplished when compiling to JavaScript."

I don't think this package really fulfills that promise at the moment. It would be nice to add some documentation about how that is handled when compiling to Javascript.

In a multi-compiler world maybe this package isn't the right place for that. In that case, perhaps we put the documentation somewhere else but it would still be nice if it existed.

[flip111]

One part of it is that bitwise operations cause a float 64 to be internally converted to int 32 by JS https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Expressions_and_operators#bitwise_operators

The rest (other than bitwise operators) is also unclear to me, i was wondering about it as well.