fix(dev): skip Vanilla Extract plugin if not installed

[markdalgleish]

Closes: #6314

This ensures that, in the rare case that someone is using a .css.js/ts file extension when not using Vanilla Extract in their app, we don't inadvertently try to process it with the Vanilla Extract plugin.

[changeset-bot]

🦋 Changeset detected

Latest commit: a7422b7

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

This PR includes changesets to release 18 packages

Name	Type
@remix-run/dev	Patch
create-remix	Patch
@remix-run/css-bundle	Patch
remix	Patch
@remix-run/architect	Patch
@remix-run/cloudflare	Patch
@remix-run/cloudflare-pages	Patch
@remix-run/cloudflare-workers	Patch
@remix-run/deno	Patch
@remix-run/eslint-config	Patch
@remix-run/express	Patch
@remix-run/netlify	Patch
@remix-run/node	Patch
@remix-run/react	Patch
@remix-run/serve	Patch
@remix-run/server-runtime	Patch
@remix-run/testing	Patch
@remix-run/vercel	Patch

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[github-actions]

🤖 Hello there,

We just published version v0.0.0-nightly-c7ad951-20230508 which includes this pull request. If you'd like to take it for a test run please try it out and let us know what you think!

Thanks!

[github-actions]

🤖 Hello there,

We just published version 1.16.1-pre.0 which includes this pull request. If you'd like to take it for a test run please try it out and let us know what you think!

Thanks!

[github-actions]

🤖 Hello there,

We just published version 1.16.1 which includes this pull request. If you'd like to take it for a test run please try it out and let us know what you think!

Thanks!