Control the `useAsyncify` setting per module instantiation

[radu-matei]

When instantiating a linker with Asyncify enabled, all modules linked and instantiated with the linker will be instantiated using Asyncify's JavaScript wrapper.

We should allow a per-instance (and by extension per module linked) setting for Asyncify.

A current workaround is instantiating a module separately and adding it to the linker using the instance method.