[Feature]: Support for "fastly" serverBuildTarget

[harmony7]

At Fastly we'd like to get support for Fastly Compute@Edge added to Remix.

We've done some work and we've been able to build both a runtime (implements the stuff in @remix-run/server-runtime) and a server adapter (the part that is called by user code). We've also been able to build a template that can be used to kickstart an app with remix create. We'll be releasing them soon under Fastly Labs.

Fastly Compute@Edge's JavaScript runtime works using the Service Worker API model, similar to the approach taken by Cloudflare Workers. For this reason, using a serverBuildTarget value of cloudflare-workers works for now. So I'd like to contribute code to add fastly-compute-js as one of the values of serverBuildTarget: 1) when building an appropriate esbuild config; 2) to get serverBareModulesPlugin to bundle everything.

EDIT: runtime/adapter/template will be maintained by Fastly, we only need to get the serverBuildTarget added.

[harmony7]

Related: #2888

[harmony7]

Have created this PR
#4860

Once this is accepted, it will be possible to use the adapter we created at
https://github.com/fastly/remix-compute-js

[MichaelDeBoey]

#4841 is now merged, which will provide new extra config options (serverConditions, serverMainFields & serverMinify) that can be used to implement this instead