Parse models without `type: object` as objects if they contain properties?

[danmichaelo]

Follow-up from https://github.com/ferdikoomen/openapi-typescript-codegen/issues/1178#issuecomment-1680690220 :

It seems like most OpenAPI tooling supports objects to be defined implicitly, without type: object. What do you think about supporting that in openapi-typescript-codegen too?

I've tried to dig into the JSON Schema spec to find out what's the correct way to handle this case, but unsuccessfully.

I'm not sure if this PR is complete, but I've created it to start the discussion of whether implicit objects should be supported or not.