Revert "breaking: replace EnvVarConfig.static with EnvVarConfig.availability"

[Rich-Harris]

Reverts #16267 because there are still some outstanding questions about the API, and we don't want to block #16282 on it

[pkg-svelte-dev]

Install the latest version of @sveltejs/kit from 5210967:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/52109674ddb52c7b9db214ee7b735d343fb9f4c2

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/16335

[changeset-bot]

⚠️ No Changeset found

Latest commit: 5210967

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-kit-16335-svelte.vercel.app/