feat: allow plugin users to create a ClientUI

[nicdumz]

Greetings,

I would like to suggest an alternative to #591 which will allow unblocking the consumer getsops/sops#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.

[nicdumz]

And now with 💯 passing tests, sorry.

[nicdumz]

FWIW, I saw that you were working on a plugin feature branch, and main still merges easily with your plugin after this, see nicdumz@c8259ad if you're interested.