feat(api): broadcast events to ALL channels #28487

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()`.

test/functional/vimscript/api_functions_spec.lua

@@ -181,8 +181,8 @@ describe('eval-API', function()
     eq('Vim(call):E117: Unknown function: buffer_get_line', err)
 
     -- some api functions are only useful from a msgpack-rpc channel
-    err = exc_exec('call nvim_subscribe("fancyevent")')
-    eq('Vim(call):E117: Unknown function: nvim_subscribe', err)
+    err = exc_exec('call nvim_set_client_info()')
+    eq('Vim(call):E117: Unknown function: nvim_set_client_info', err)
   end)
 
   it('have metadata accessible with api_info()', function()