feat(tui): add nvim_ui_send (#35406)

This function allows the Nvim core to write arbitrary data to a TTY
connected to a UI's stdout.

runtime/lua/vim/ui/clipboard/osc52.lua

@@ -14,8 +14,7 @@ function M.copy(reg)
   return function(lines)
     local s = table.concat(lines, '\n')
     -- The data to be written here can be quite long.
-    -- Use nvim_chan_send() as io.stdout:write() doesn't handle EAGAIN. #26688
-    vim.api.nvim_chan_send(2, osc52(clipboard, vim.base64.encode(s)))
+    vim.api.nvim_ui_send(osc52(clipboard, vim.base64.encode(s)))
   end
 end
 
@@ -34,7 +33,7 @@ function M.paste(reg)
       end,
     })
 
-    io.stdout:write(osc52(clipboard, '?'))
+    vim.api.nvim_ui_send(osc52(clipboard, '?'))
 
     local ok, res