Implement support for turning .json files into Lua modules (#308)

* Stub implementation

* Flesh out feature and add tests. Other snapshots currently failing.

* Blacklist .meta.json in JSON handler

* Write to correct property (Source) instead of Value

* Update changelog

src/snapshot_middleware/snapshots/librojo__snapshot_middleware__json__test__instance_from_vfs.snap

@@ -0,0 +1,20 @@
+---
+source: src/snapshot_middleware/json.rs
+expression: instance_snapshot
+---
+snapshot_id: ~
+metadata:
+  ignore_unknown_instances: false
+  instigating_source:
+    Path: /foo.json
+  relevant_paths:
+    - /foo.json
+    - /foo.meta.json
+  context: {}
+name: foo
+class_name: ModuleScript
+properties:
+  Source:
+    Type: String
+    Value: "return {\n\t[\"1invalidident\"] = \"nice\",\n\tarray = {1, 2, 3},\n\t[\"false\"] = false,\n\tfloat = 1234.5452,\n\tint = 1234,\n\tnull = nil,\n\tobject = {\n\t\thello = \"world\",\n\t},\n\t[\"true\"] = true,\n}"
+children: []