fix: shared_state initialization was undefined instead of empty object.
refactor: Better send_error checks, send / print stack traces.

No more `err.constructor === Object` checking. Check for `err.stack` presence instead and send / print it.
refactor: Simplify merge strategy resolution.

Check if special left<-right combination exists and use it. Default to replace_data(left, right) otherwise.
Change --wysia-subdir default from 'wysia/' to './'.