Add Remix 3 export subpaths and optional peer deps #24662

Prepare @sentry/remix to hold Remix 3 support. No instrumentation yet.

• Make the four Remix 2 peer deps optional. They are required today, so npm would auto install React into a Remix 3 app.
• Add remix 3 as an optional peer dep.
• Add the Remix 3 export subpaths, pointing at stub entries.
• Add the new rollup entry points.
• Register the Remix 3 e2e app in the CI matrix, so the next pull request does not mix new instrumentation with new CI surface.

engines stays as it is. remix already requires Node 24.3 or newer, and raising ours would break Remix 2 users.

Done when the package builds, both e2e apps run in CI, and no Remix 2 behaviour changes.

Subpath names settled: @sentry/remix/v3, /v3/client, /v3/node.

/v3/dc-shim moved to #24677. A stub shim here would be a file with no caller, and the name only matters once the browser code transform imports it by URL.