Disambiguate camelCase and PascalCase in *.meta.json and *.model.json (#563)

* Disambiguate camelCase and PascalCase.

*.meta.json forces camelCase while *.model.json forces PascalCase. This commit reinforces camelCase as the preference for both, but allows for PascalCase in both as well.

* Made requested changes, breaking due to serde bug.

* Make work with existing Serde stuff

* Work around MSRV

Co-authored-by: Lucien Greathouse <me@lpghatguy.com>

src/snapshot_middleware/snapshots/librojo__snapshot_middleware__json_model__test__model_from_vfs_legacy.snap

@@ -0,0 +1,29 @@
+---
+source: src/snapshot_middleware/json_model.rs
+assertion_line: 186
+expression: instance_snapshot
+---
+snapshot_id: ~
+metadata:
+  ignore_unknown_instances: false
+  instigating_source:
+    Path: /foo.model.json
+  relevant_paths:
+    - /foo.model.json
+  context: {}
+name: foo
+class_name: IntValue
+properties:
+  Value:
+    Int64: 5
+children:
+  - snapshot_id: ~
+    metadata:
+      ignore_unknown_instances: false
+      relevant_paths: []
+      context: {}
+    name: The Child
+    class_name: StringValue
+    properties: {}
+    children: []
+