Add an API-stable path to accessing the private env globally

[AndreLouisIssa]

Currently you can access the private envs by doing envy.locals.private[public_env] but anything under the envy.locals path is unstable.
Normally, that's ok because private envs themselves are unstable, but there may be times where a mod wants to access its own private env from an external location, such as with Hades 2, from a lovely patch in the base game scripts.

Additionally, a RoM-aware helper could be envy.private[plugin_guid] (or maybe mods_private as an extra) so having a handle on the public env first isn't necessary
this would be helpful for accessing a private function from a global path in rom.game