refactor: introduce nvim_on internally #39883

Problem:
`nvim_create_autocmd` is too verbose and its `callback` requires extra
"nesting".

Solution:
Introduce `nvim_on`. Start using it internally. Then we can get a feel
for how it should look before making it public.

runtime/lua/vim/tty.lua

@@ -28,18 +28,19 @@ function M.request(payload, opts, on_response)
     timer = assert(vim.uv.new_timer())
   end
 
-  local id = vim.api.nvim_create_autocmd('TermResponse', {
-    group = opts.group,
-    nested = true,
-    callback = function(ev)
+  local id = require('vim._core.util').nvim_on(
+    'TermResponse',
+    opts.group,
+    { nested = true },
+    function(ev)
       local stop = on_response(ev.data.sequence)
       -- If on_response is done, cancel the timeout so on_timeout doesn't fire spuriously.
       if stop and timer and not timer:is_closing() then
         timer:close()
       end
       return stop
-    end,
-  })
+    end
+  )
 
   if payload ~= '' then
     vim.api.nvim_ui_send(payload)