Add root level export for wrappers_pb

[steven-supersolid]

To access the wrappers objects a client must do the following:

const jspbWrappers = require('google-protobuf/google/protobuf/wrappers_pb');

This requires knowledge of the internal structure of the package. Instead a client should be able to do something like

const jspbWrappers = require('google-protobuf').jspbWrappers

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Due to our limited support resources, feature requests will generally not be accepted, except for those referring to build or tooling integration.

[dibenede]

Sorry for the delay. I think I understand where this sentiment is coming from, but I think the path to package approach is typical across protobuf languages.