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()`.

src/nvim/api/deprecated.c

@@ -786,3 +786,23 @@ theend:
   api_clear_error(&nested_error);
   return rv;
 }
+
+/// @deprecated
+///
+/// @param channel_id Channel id (passed automatically by the dispatcher)
+/// @param event      Event type string
+void nvim_subscribe(uint64_t channel_id, String event)
+  FUNC_API_SINCE(1) FUNC_API_REMOTE_ONLY
+{
+  // Does nothing. `rpcnotify(0,…)` broadcasts to all channels, there are no "subscriptions".
+}
+
+/// @deprecated
+///
+/// @param channel_id Channel id (passed automatically by the dispatcher)
+/// @param event      Event type string
+void nvim_unsubscribe(uint64_t channel_id, String event)
+  FUNC_API_SINCE(1) FUNC_API_REMOTE_ONLY
+{
+  // Does nothing. `rpcnotify(0,…)` broadcasts to all channels, there are no "subscriptions".
+}