Add timeout to list_r_sessions

[shikokuchuo]

Just a quick PR:

1. Adds a 5s timeout so the client doesn't hang when querying. This could conceivably happen if code in a particular session is busy and not at the top level / calling later::run_now() for the promise to execute.

Tagging on some other minor changes:

2. Some cosmetic tidy-ups - to refer to 'session' instead of 'i' during session selection, which I think is slightly nicer in the UI.
3. Use the best platform-specific IPC transport (this needs to be onLoad so it's not saved when the package is built - it basically follows what mirai uses).
4. Avoids suppressWarnings(), now I've made that possible.

[simonpcouch]

🕛🕐