Add support for updating instances defined in project nodes

src/snapshots/serve_session__change_file_in_project_after.snap

@@ -0,0 +1,36 @@
+---
+source: src/serve_session.rs
+expression: "view_tree(&session.tree(), &mut redactions)"
+---
+id: id-1
+name: change_file_in_project
+class_name: Folder
+properties: {}
+metadata:
+  ignore_unknown_instances: true
+  instigating_source:
+    Path: /foo/default.project.json
+  relevant_paths:
+    - /foo/default.project.json
+children:
+  - id: id-2
+    name: Child
+    class_name: StringValue
+    properties:
+      Value:
+        Type: String
+        Value: Changed!
+    metadata:
+      ignore_unknown_instances: false
+      instigating_source:
+        ProjectNode:
+          - Child
+          - class_name: ~
+            children: {}
+            properties: {}
+            ignore_unknown_instances: ~
+            path: "/foo\\file.txt"
+      relevant_paths:
+        - /foo/file.txt
+        - /foo/file.meta.json
+    children: []

src/snapshots/serve_session__change_file_in_project_before.snap

@@ -0,0 +1,36 @@
+---
+source: src/serve_session.rs
+expression: "view_tree(&session.tree(), &mut redactions)"
+---
+id: id-1
+name: change_file_in_project
+class_name: Folder
+properties: {}
+metadata:
+  ignore_unknown_instances: true
+  instigating_source:
+    Path: /foo/default.project.json
+  relevant_paths:
+    - /foo/default.project.json
+children:
+  - id: id-2
+    name: Child
+    class_name: StringValue
+    properties:
+      Value:
+        Type: String
+        Value: initial content
+    metadata:
+      ignore_unknown_instances: false
+      instigating_source:
+        ProjectNode:
+          - Child
+          - class_name: ~
+            children: {}
+            properties: {}
+            ignore_unknown_instances: ~
+            path: "/foo\\file.txt"
+      relevant_paths:
+        - /foo/file.txt
+        - /foo/file.meta.json
+    children: []

src/snapshots/serve_session__change_file_in_project_patch.snap

@@ -0,0 +1,16 @@
+---
+source: src/serve_session.rs
+expression: redactions.redacted_yaml(result)
+---
+- 1
+- - removed: []
+    added: []
+    updated:
+      - id: id-2
+        changed_name: ~
+        changed_class_name: ~
+        changed_properties:
+          Value:
+            Type: String
+            Value: Changed!
+        changed_metadata: ~