CI: add Makefile target to verify default highlighting groups are present

When adding new highlight groups, one needs to make sure to also add a
"default link NewHlGroup ExistingHlGroup" in highlight.c code, so that
when resetting a color scheme the old color won't be left behind.

So add a Makefile in the 'ci' directory that verifies that all
documented '*hl-<groupname>' from the documentation are either reflected
in the source code, or belong to a list of 'known to be ignored'
highlight groups and let that check run as part of the CI test suite.

related: #16676
closes: #16678

Signed-off-by: Christ van Willegen <cvwillegen@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

ci/hlgroups.ignore

@@ -0,0 +1,66 @@
+ColorColumn
+ComplMatchIns
+Conceal
+Cursor
+CursorColumn
+CursorIM
+CursorLine
+CursorLineNr
+debugBreakpoint
+debugPC
+DiffAdd
+DiffChange
+DiffDelete
+DiffText
+Directory
+ErrorMsg
+FoldColumn
+Folded
+Ignore
+IncSearch
+lCursor
+LineNr
+LineNrAbove
+LineNrBelow
+MatchParen
+Menu
+ModeMsg
+MoreMsg
+MsgArea
+NonText
+Normal
+Pmenu
+PmenuSbar
+PmenuSel
+PmenuThumb
+Question
+Scrollbar
+Search
+SignColumn
+SpecialKey
+SpellBad
+SpellCap
+SpellLocal
+SpellRare
+StatusLine
+StatusLineNC
+StatusLineTerm
+StatusLineTermNC
+TabLine
+TabLineFill
+TabLineSel
+Terminal
+Title
+TOhtmlProgress
+TOhtml-progress-color
+ToolbarButton
+ToolbarLine
+Tooltip
+User1
+User1..9
+User9
+VertSplit
+Visual
+VisualNOS
+WarningMsg
+WildMenu