feat: Add privatePrefix for private environment variables

[tcc-sejohnson]

Closes #9776.

Adds a new config option, config.kit.env.privatePrefix, for setting a private prefix on environment variables. This defaults to ''.

To prevent super-weird and unexpected behaviors, the logic is:

• private: does not begin with public prefix, does begin with private prefix
• public: does begin with public prefix, does not begin with private prefix

This has the side benefit of not allowing the two prefixes to be the same.

Note: Had to create env.js utils file so that the utils could be imported into server/index -- putting them pretty much anywhere else ended up causing a transitive dependency on some node package somewhere that wasn't compatible with Node 16.

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 pnpm changeset and following the prompts. Changesets that add features should be minor and those that fix bugs should be patch. Please prefix changeset messages with feat:, fix:, or chore:.

[changeset-bot]

🦋 Changeset detected

Latest commit: 5a78adc

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

This PR includes changesets to release 1 package

Name	Type
@sveltejs/kit	Minor

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]

just a heads up, there's a merge conflict on this one

[tcc-sejohnson]

Weird; thanks