feat(api): "broadcast" events to ALL channels

[justinmk]

Problem:

vim.rpcnotify(0) and rpcnotify(0) are documented as follows:

If {channel} is 0, the event is broadcast to all channels.

But that's not actually true. Channels must call nvim_subscribe to receive "broadcast" events, so it's actually "multicast".

• Assuming there is a use-case for "broadcast", the current model adds an extra step for broadcasting: all channels need to "subscribe".
• The presence of nvim_subscribe is a source of confusion for users, because its name implies something more generally useful than what it does.

Presumably the use-case of nvim_subscribe is to avoid "noise" on RPC channels not expected a broadcast notification, and potentially an error if the channel client reports an unknown event.

Solution:

• Deprecate nvim_subscribe/nvim_unsubscribe.
  • If applications want to multicast, they can keep their own multicast list. Or they can use nvim_list_chans() and nvim_get_chan_info() to enumerate and filter the clients they want to target.
• Always send "broadcast" events to ALL channels. Don't require channels to "subscribe" to receive broadcasts. This matches the documented behavior of rpcnotify().