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

test/functional/legacy/051_highlight_spec.lua

@@ -24,10 +24,10 @@ describe(':highlight', function()
                          guifg=Blue      |
       EndOfBuffer    xxx links to NonText|
                                          |
+      TermCursor     xxx cterm=reverse   |
+                         gui=reverse     |
+      TermCursorNC   xxx cleared         |
       NonText        xxx ctermfg=12      |
-                         gui=bold        |
-                         guifg=Blue      |
-      Directory      xxx ctermfg=4       |
       -- More --^                         |
     ]])
     feed('q')

test/functional/terminal/cursor_spec.lua

@@ -129,10 +129,8 @@ describe('cursor with customized highlighting', function()
 
   before_each(function()
     clear()
-    nvim('set_var', 'terminal_focused_cursor_highlight', 'CursorFocused')
-    nvim('set_var', 'terminal_unfocused_cursor_highlight', 'CursorUnfocused')
-    nvim('command', 'highlight CursorFocused ctermfg=45 ctermbg=46')
-    nvim('command', 'highlight CursorUnfocused ctermfg=55 ctermbg=56')
+    nvim('command', 'highlight TermCursor ctermfg=45 ctermbg=46 cterm=NONE')
+    nvim('command', 'highlight TermCursorNC ctermfg=55 ctermbg=56 cterm=NONE')
     screen = Screen.new(50, 7)
     screen:set_default_attr_ids({
       [1] = {foreground = 45, background = 46},

test/functional/terminal/helpers.lua

@@ -34,6 +34,8 @@ local function disable_mouse() feed_termcode('[?1002l') end
 
 
 local function screen_setup(extra_height)
+  nvim('command', 'highlight TermCursor cterm=reverse')
+  nvim('command', 'highlight TermCursorNC ctermbg=11')
   nvim('set_var', 'terminal_scrollback_buffer_size', 10)
   if not extra_height then extra_height = 0 end
   local screen = Screen.new(50, 7 + extra_height)