Hide pieces of the API

There's various pieces of the API that should probably be hidden or made non-constructable.
Hiding functions would allow us to later replace or alter the code behind them without breaking 'good' plugins because they shouldn't be using them directly. Making some structures non-constructable would let us add fields in the future easily.

• handle_rpc is an internal function, it should be #[doc(hidden)].
• Ideally users should never have to use object_from_stdin/object_from_stdout. These should probably be internal, and we just pass the requests/notifications to the plugin.
• parse_stdin should be hidden
• Http should be made non-constructable (add a hidden empty marker field), just in case.
• VoltEnvironment should be made non-constructable.
  • Really I think VoltEnvironment should have to be created if you want to use it.
  • Would let us cache values in the future.
• PluginError should perhaps be #[non_exhaustive] in case we want to add other error kinds with types in the future.

Locate the public definitions and exports for handle_rpc, object_from_stdin, object_from_stdout, parse_stdin, Http, VoltEnvironment, and PluginError. Review how plugins currently use these APIs and determine the intended visibility, construction, and future-compatibility changes; done means the listed APIs enforce those constraints without breaking supported plugin usage.