bug#64177: outline-minor-mode-highlight fails to activate for custom headings

[Juri Linkov]

>> but many modes override this default with own function that ignores
>> outline-heading-alist:
>>
>> lisp-outline-level, c-outline-level, cperl-outline-level, perl-outline-level,
>> tcl-outline-level, cfengine2-outline-level, conf-outline-level,
>> nroff-outline-level, latex-outline-level
>>
>> - all these ignore outline-heading-alist.
>
> In the modes that override the default, outline-level could be
> set up to use outline-heading-alist, avoiding the conditional.

Patches are welcome to support outline-heading-alist in the
aforementioned functions.