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.

test/functional/ui/options_spec.lua

@@ -23,6 +23,7 @@ describe('UI receives option updates', function()
       mousemoveevent=false,
       showtabline=1,
       termguicolors=false,
+      termsync=true,
       ttimeout=true,
       ttimeoutlen=50,
       verbose=0,