feat(api): pass 0 to nvim_get_chan_info for current channel (#27321)

Getting current channel info was kind of annoying via RPC. Two
functions had to be called:
1. `nvim_get_api_info` which returns `[channel_id, meta_data]`.
  - This results in `channel_id = api.nvim_get_api_info()[0]`.
  - Here the meta_data is sent but never used.
2. Finally call `nvim_get_chan_info(channel_id)`.

This commit reduces the need for `nvim_get_api_info` as passing 0
returns current channel info.

runtime/doc/api.txt

@@ -888,6 +888,9 @@ nvim_get_api_info()                                      *nvim_get_api_info()*
 nvim_get_chan_info({chan})                              *nvim_get_chan_info()*
     Gets information about a channel.
 
+    Parameters: ~
+      • {chan}  channel_id, or 0 for current channel
+
     Return: ~
         Dictionary describing a channel, with these keys:
         • "id" Channel id.

runtime/doc/news.txt

@@ -405,6 +405,8 @@ The following changes to existing APIs or features add new behavior.
 • Attempting to set an invalid keycode option (e.g. `set t_foo=123`) no longer
   gives an error.
 
+• Passing 0 to |nvim_get_chan_info()| gets info about the current channel.
+
 • |:checkhealth| buffer can now be opened in a split window using modifiers like
   |:vertical|, |:horizontal| and |:botright|.