feat(tui): restore 'ttyfast' to control tty requests #38699

Problem:
When running nvim on a remote machine over SSH, if there is high ping,
then bg detection may not complete in time. This results in a warning
every time nvim is started. #38648

Solution:
Restore 'ttyfast' option and allow it to control whether or not bg
detection is performed. Because this is during startup and before any
user config or commands, we use the environment variable
`NVIM_NOTTYFAST` to allow disabling `ttyfast` during initialization.

src/nvim/options.lua

@@ -9777,12 +9777,20 @@ local options = {
     {
       abbreviation = 'tf',
       defaults = true,
+      desc = [=[
+        Assume that the underlying terminal can respond quickly to queries
+        required by features such as 'background' detection.
+        
+        Nvim issues terminal queries before reading the user's |config| file,
+        so disabling this option there will not work. Set $NVIM_NOTTYFAST
+        before starting Nvim to disable terminal queries.
+      ]=],
       full_name = 'ttyfast',
       no_mkrc = true,
       scope = { 'global' },
-      short_desc = N_('Deprecated'),
+      short_desc = N_('assume terminal responds quickly, enabling more features'),
       type = 'boolean',
-      immutable = true,
+      varname = 'p_tf',
     },
     {
       abbreviation = 'udir',