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/ops.c

@@ -45,7 +45,7 @@
 #include "nvim/screen.h"
 #include "nvim/search.h"
 #include "nvim/strings.h"
-#include "nvim/term.h"
+#include "nvim/ui.h"
 #include "nvim/undo.h"
 #include "nvim/window.h"
 #include "nvim/os/input.h"
@@ -3243,7 +3243,7 @@ void ex_display(exarg_T *eap)
       }
       if (n > 1 && yb->y_type == MLINE)
         MSG_PUTS_ATTR("^J", attr);
-      out_flush();                          /* show one line at a time */
+      ui_flush();                          /* show one line at a time */
     }
     os_breakcheck();
   }