feat: allow plugin users to create a `ClientUI`#611

Greetings, I would like to suggest an alternative to https://github.com/FiloSottile/age/pull/591 which will allow unblocking the consumer https://github.com/getsops/sops/pull/1641. (I'm interested in plugin support in `sops` so I can try out FIDO2 keys inside of Nix via `sops-nix`). Instead of exposing all `age` internals into a separate package (which I presume isn't the best from a maintenance PoV), I move functions which need to be shared between `plugin` and `main` into two new internal private modules. This lets me eventually expose `NewClientUI()` for consumers. - **Move logging functions to internal/logger.** - **Move terminal related functions to internal/term** - **Expose a public `plugin.NewClientUI()`** Kudos to @brianmcgee for the original work. @FiloSottile I'm happy to iterate on this code, let me know your constraints and I'll work with you. HTH.