feat(tui): add 'termsync' option (#25871)

The 'termsync' option enables a mode (provided the underlying terminal
supports it) where all screen updates during a redraw cycle are buffered
and drawn together when the redraw is complete. This eliminates tearing
or flickering in cases where Nvim redraws slower than the terminal
redraws the screen.

src/nvim/options.lua

@@ -8809,6 +8809,21 @@ return {
       type = 'string',
       varname = 'p_tpf',
     },
+    {
+      defaults = { if_true = true },
+      desc = [=[
+        If the host terminal supports it, buffer all screen updates
+        made during a redraw cycle so that each screen is displayed in
+        the terminal all at once. This can prevent tearing or flickering
+        when the terminal updates faster than Nvim can redraw.
+      ]=],
+      full_name = 'termsync',
+      redraw = { 'ui_option' },
+      scope = { 'global' },
+      short_desc = N_('synchronize redraw output with the host terminal'),
+      type = 'bool',
+      varname = 'p_termsync',
+    },
     {
       defaults = { if_true = false },
       full_name = 'terse',