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

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>