- this modifies `isChildDoc` to always return false if child docs are
   not possible according to the schema, and restores part of the
   logic of `parseObjectFieldValue` so that it can handle
   multiple modifiers
 - re-enables the ability to use multiple modifiers in a single
   update, like `{"add": "foo", "remove": "bar"}`
 - re-enables the ability to have a field with a name like
   `set` or `remove` and be able to use updates like
   `{"set": {"set": "foo"}`
 - both of these are possible now but only if the schema
   doesn't support child/nested docs; if the schema does
   support them, then functionality is unchanged and either
   will continue throwing an exception

 - Map.of doesn't allow null values, so there's still one
   remaining use of HashMap

…teJsonTest.java