Use `camelCase` instead of `kebab_case` in spec for consistency

Currently, the root properties of the spec use kebab_case for properties e.g. semantic_types and chart_spec while everywhere else the spec uses camelCase for properties e.g. chartType. This is inconsistent not only within the schema, but also with the usual convention for JavaScript/TypeScript which is to use camelCase (I understand kebab_case is the standard in e.g. python), but having both is definitively odd and confusing.

{
-  "semantic_types": {
+  "semanticTypes": {
     "count": "Quantity",
     "percentage": "Percentage",
     "region": "Category"
   },
-  "chart_spec": {
+  "chartSpec": {
     "chartType": "Scatter Plot",
     "encodings": {
       "x": "count",
       "y": "percentage",
       "color": "region"
     }
   },
   "data": {}
 }

Maybe I am missing something, there must be a valid reason? Additionally, we could maybe rename chartSpec to simply spec?

In any case, standardizing this to use camelCase for all properties would be more consistent and standard.