Update the `groups-max-layers` rule

[HugoDaniel]

On the groups-max-layers rule:

1. change the option name from skipClasses to ignore
2. clarify plural usage (either implement plural support or change the doc/option to specify which names are valid on the array)
3. report an error when the skipClasses array includes invalid/unsupported class values

Currently the documentation says:

[jedrichards]

1. I think ignore is too generic, since we may want to use that in future for a more general option. Since only developers will likely look at this rule I think skipClasses is probably ok.
2. I think the plural usage is just a mistake in the docs
3. Good idea we might be able to use the new ClassValue enum in the file format ts types to do this validation in the rule

[christianklotz]

Indeed, I believe we originally started with having it called ignore. Also to avoid confusion with the upcoming ignore mechanism, skipClasses still makes sense. If there'll ever be a more designer facing part to this it could be solved by using a more designer friendly title for this option, e.g. Skip or similar.