Reinstate ProcessContext provider

[michaelbromley]

Is your feature request related to a problem? Please describe.
After the v1.0 refactor of the worker architecture, we no longer have a means to determine whether code is being run in the context of the server or worker. We previously had the ProcessContext provider which allowed us to add conditional checks like

if (this.processContext.isServer) {
  // ...
}

Describe the solution you'd like
Reinstate the ProcessContext, using the same implementation as before.