Support ES 6 default exports for form builder field JavaScript data handlers

Support ES 6 default exports for form builder field JavaScript data handlers

For legacy exports the value loaded by `require()` (`FormBuilderField`) will be
whatever the module author returned within the module definition. ES 6 modules
on the other hand will be passed as an object containing the exported values,
with the default export residing in the `default` key.

tslib's `__importDefault` will transform a legacy export into a ES 6 module, by
putting the exported value into the `default` key of an freshly created object.

This allows us to handle both legacy as well as ES 6 default exports identically
by simply using the `default` value of the `FormBuilderField` variable.