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/spell.c

@@ -325,7 +325,7 @@
 #include "nvim/search.h"
 #include "nvim/strings.h"
 #include "nvim/syntax.h"
-#include "nvim/term.h"
+#include "nvim/ui.h"
 #include "nvim/tempfile.h"
 #include "nvim/undo.h"
 #include "nvim/os/os.h"
@@ -5476,7 +5476,7 @@ static int spell_read_dic(spellinfo_T *spin, char_u *fname, afffile_T *affile)
       msg_clr_eos();
       msg_didout = FALSE;
       msg_col = 0;
-      out_flush();
+      ui_flush();
     }
 
     // Store the word in the hashtable to be able to find duplicates.
@@ -6327,7 +6327,7 @@ static int tree_add_word(spellinfo_T *spin, char_u *word, wordnode_T *root, int
       msg_clr_eos();
       msg_didout = FALSE;
       msg_col = 0;
-      out_flush();
+      ui_flush();
     }
 
     // Compress both trees.  Either they both have many nodes, which makes
@@ -7749,7 +7749,7 @@ static void spell_message(spellinfo_T *spin, char_u *str)
     if (!spin->si_verbose)
       verbose_enter();
     MSG(str);
-    out_flush();
+    ui_flush();
     if (!spin->si_verbose)
       verbose_leave();
   }