Move some serve code into serve_session.rs, start writing serve session tests

src/snapshots/serve_session__just_folder.snap

@@ -0,0 +1,15 @@
+---
+source: src/serve_session.rs
+expression: tree_value
+---
+id: id-1
+name: foo
+class_name: Folder
+properties: {}
+metadata:
+  ignore_unknown_instances: false
+  instigating_source:
+    Path: /foo
+  relevant_paths:
+    - /foo
+children: []