feat: optional entry.{client,server} for react 17

[mcansh]

Closes: #5669
Closes: #5810

• Docs
• Tests

Testing Strategy:

[changeset-bot]

🦋 Changeset detected

Latest commit: e249cd5

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

This PR includes changesets to release 18 packages

Name	Type
remix	Patch
@remix-run/dev	Patch
create-remix	Patch
@remix-run/css-bundle	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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[ZackWard]

@mcansh I just discovered this change (optional entry files) today and found it pretty confusing. I would like to propose that along with this change, the prominent docs/file-conventions/entry.client.md and docs/file-conventions/entry.server.md files be updated to clarify that users should not expect to see these files by default.

[github-actions]

🤖 Hello there,

We just published version v0.0.0-nightly-d6c9737-20230322 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!