make env vars a bit easier to understand/follow in adapter-node

[Rich-Harris]

I find the environment variable options in adapter-node confusing in the extreme — they're options that set variables that allow you to set variables (that tell the app how to read variables, in the case of headers). The least we can do is avoid giving them inconsistent aliases on top of that.

Please don't delete this checklist! Before submitting the PR, please make sure you do the following:

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests

• Run the tests with pnpm test and lint the project with pnpm lint and pnpm check

Changesets

• If your PR makes a change that should be noted in one or more packages' changelogs, generate a changeset by running pnpx changeset and following the prompts. All changesets should be patch until SvelteKit 1.0

[changeset-bot]

🦋 Changeset detected

Latest commit: 8c68eb0

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@sveltejs/adapter-node	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[benmccann]

would something like envAliases make sense as a name?

[Rich-Harris]

i feel like to be properly accurate it would need to be envVarAliases which seems like a step too far

[benmccann]

50 lines of code removed. awesome!