assistant: Restructure ambient context in preparation for adding more (#11822)

This PR restructures the ambient context in the `assistant` crate to
make it more amenable to adding more kinds of ambient context.

Release Notes:

- N/A

crates/assistant/src/assistant.rs

@@ -1,3 +1,4 @@
+mod ambient_context;
 pub mod assistant_panel;
 pub mod assistant_settings;
 mod codegen;