[Feature]: Add better support for custom environments

[harmony7]

What is the new or updated feature that you are suggesting?

Currently, @remix-run/dev checks serverBuildTarget set in RemixConfig against some known hardcoded string values to make certain decisions.

It appears there has been some history here:

• In the past there have been config values in RemixConfig such as serverModuleFormat and serverPlatform, but now these have been deprecated and they are now implied by the serverBuildTarget value, which is the preferred way
• More recently-added decisions do not have representations in the base RemixConfig at all: for example, serverBareModulesPlugin checks serverBuildTarget directly against a few hardcoded values as one of the conditions under which to "bundle everything"; and the compilerServer also checks serverBuildTarget directly to decide whether to include "browser" as one of the main fields to use for referenced modules.

I'd like to suggest that more of these settings be added to RemixConfig and be implied by serverBuildTarget so that it becomes possible to target environments that @remix-run/dev doesn't currently know about. The preferred approach can still be to use serverBuildTarget, but allow the user to leave this blank and provide a custom set of values if they know how their environment would want them.

NOTE: I'm not suggesting that Remix expose esbuild config directly. This is NOT the same as #2153.

Why should this feature be included?

I love Remix's "works-with-your-runtime" philosophy, so it would be great if it's possible to build for custom environments without @remix-run/dev having to know about them individually.

[harmony7]

Looks like this is being taken care of in #4841