Add CWD, PID and TTY to PaneInfo to make it available to plugins

[msirringhaus]

Notes:

• Instead of sending the PaneManifest directly from screen to the plugin, we proxy it through Pty, which then fills out runtime-specific values (tests need to include this additional hop now, too)
• TTY-detection is currently only available on Linux. It probably would make sense to separate this out into it's own crate (this would also make it easier for contributors to add e.g. macos). Let me know, if that is something I should do.
• protobuf can't handle PathBufs, so we have to convert them to Strings.

[msirringhaus]

I didn't know about #3765 before starting this. But I think this PR includes most of it, too.