Add new highlight groups TermCursor/TermCursorNC

These highlight groups replace the old mechanism of setting:

  - {g,b}:terminal_focused_cursor_highlight
  - {g,b}:terminal_unfocused_cursor_highlight

src/nvim/syntax.c

@@ -5785,6 +5785,8 @@ static char *(highlight_init_both[]) =
       "TabLineSel term=bold cterm=bold gui=bold"),
   CENT("TabLineFill term=reverse cterm=reverse",
       "TabLineFill term=reverse cterm=reverse gui=reverse"),
+  CENT("TermCursor cterm=reverse",
+      "TermCursor cterm=reverse gui=reverse"),
   NULL
 };