feat(tui): graduate the +tui feature

This was previously disabled due to build issues on windows.
Any reasonable platform can now be expected to have the necessary
interfaces to build and run the TUI subsystem.

Runtime quality issues of using the TUI (on any new platform) are not
relevant here. Just run Nvim in an external UI instead of the TUI as always.

src/nvim/version.c

@@ -69,11 +69,7 @@ static char *features[] = {
   "-iconv",
 #endif
 
-#ifdef FEAT_TUI
   "+tui",
-#else
-  "-tui",
-#endif
   NULL
 };