refactor: Remove term modules and termcap options

- Removed term.c, term.h and term_defs.h
- Tests for T_* values were removed. screen.c was simplified as a
  consequence(the best strategy for drawing is implemented in the UI layer)
- Redraw functions now call ui.c functions directly. Updates are flushed with
  `ui_flush()`
- Removed all termcap options(they now return empty strings for compatibility)
- &term/&ttybuiltin options return a constant value(nvim)
- &t_Co is still available, but it mirrors t_colors directly
- Remove cursor tracking from screen.c and the `screen_start` function. Now the
  UI is expected to maintain cursor state across any call, and reset it when
  resized.
- Remove unused code

src/nvim/syntax.c

@@ -44,7 +44,6 @@
 #include "nvim/screen.h"
 #include "nvim/strings.h"
 #include "nvim/syntax_defs.h"
-#include "nvim/term.h"
 #include "nvim/ui.h"
 #include "nvim/os/os.h"
 #include "nvim/os/time.h"
@@ -7394,7 +7393,7 @@ static void highlight_list_two(int cnt, int attr)
 {
   msg_puts_attr((char_u *)&("N \bI \b!  \b"[cnt / 11]), attr);
   msg_clr_eos();
-  out_flush();
+  ui_flush();
   os_delay(cnt == 99 ? 40L : (long)cnt * 50L, false);
 }