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>

crates/collab/src/llm.rs

@@ -0,0 +1,16 @@
+use std::sync::Arc;
+
+use crate::{executor::Executor, Config, Result};
+
+pub struct LlmState {
+    pub config: Config,
+    pub executor: Executor,
+}
+
+impl LlmState {
+    pub async fn new(config: Config, executor: Executor) -> Result<Arc<Self>> {
+        let this = Self { config, executor };
+
+        Ok(Arc::new(this))
+    }
+}