Refactor: Restructure collab main function to prepare for new subcommand: serve llm (#15824)

This is just a refactor that we're landing ahead of any functional
changes to make sure we haven't broken anything.

Release Notes:

- N/A

Co-authored-by: Marshall <marshall@zed.dev>
Co-authored-by: Jason <jason@zed.dev>

Procfile

@@ -1,3 +1,3 @@
-collab: RUST_LOG=${RUST_LOG:-info} cargo run --package=collab serve
+collab: RUST_LOG=${RUST_LOG:-info} cargo run --package=collab serve all
 livekit: livekit-server --dev
 blob_store: ./script/run-local-minio