Supply an AppLoadContext for dev.

[benembery]

I would like to suggest we add the ability to supply an AppLoadContext that would be consumed by remix dev.

Why?
This would be really helpful when developing locally and deploying to Cloudflare workers.

Current implementation
In my server.ts I add my wrangler vars to my context as shown below.

declare var MY_WRANGLER_VAR: string | undefined;

addEventListener(
  'fetch',
  createEventHandler({
    build,
    mode: process.env.NODE_ENV,
    getLoadContext: () => {
      return {
        MY_VAR: MY_WRANGLER_VAR,
      };
    },
  }),
);

This allows me to access MY_VAR from context in my Loaders, this is great! When developing locally these aren't set and there is no way to do this other than reading from process.env.

Suggestion
I might be oversimplifying this but if in remix.confg.js we could export an additional prop e.g. devAppLoadContext we would be able to have something like the code below, meaning the app source code no longer has to patch anything accessing process.env.

module.exports = {
  // remaning config omitted for brevity... 
  devAppLoadContext: {
    MY_VAR: process.env.MY_VAR
  }
}

P.S. Thanks for building remix, it's great!