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/_defaults.lua

@@ -839,7 +839,7 @@ do
         end,
       })
 
-      io.stdout:write('\027]11;?\007')
+      vim.api.nvim_ui_send('\027]11;?\007')
     end
 
     --- If the TUI (term_has_truecolor) was able to determine that the host
@@ -927,7 +927,7 @@ do
         local decrqss = '\027P$qm\027\\'
 
         -- Reset attributes first, as other code may have set attributes.
-        io.stdout:write(string.format('\027[0m\027[48;2;%d;%d;%dm%s', r, g, b, decrqss))
+        vim.api.nvim_ui_send(string.format('\027[0m\027[48;2;%d;%d;%dm%s', r, g, b, decrqss))
 
         timer:start(1000, 0, function()
           -- Delete the autocommand if no response was received

runtime/lua/vim/_meta/api.lua

@@ -2340,6 +2340,14 @@ function vim.api.nvim_tabpage_set_var(tabpage, name, value) end
 --- @param win integer `window-ID`, must already belong to {tabpage}
 function vim.api.nvim_tabpage_set_win(tabpage, win) end
 
+--- Sends arbitrary data to a UI.
+---
+--- This sends a "ui_send" event to any UI that has the "stdout_tty" `ui-option` set. UIs are
+--- expected to write the received data to a connected TTY if one exists.
+---
+--- @param content string Content to write to the TTY
+function vim.api.nvim_ui_send(content) end
+
 --- Calls a function with window as temporary current window.
 ---
 ---

runtime/lua/vim/termcap.lua

@@ -71,7 +71,7 @@ function M.query(caps, cb)
 
   local query = string.format('\027P+q%s\027\\', table.concat(encoded, ';'))
 
-  io.stdout:write(query)
+  vim.api.nvim_ui_send(query)
 
   timer:start(1000, 0, function()
     -- Delete the autocommand if no response was received

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