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