validation of VariationRegionList axis_count

We should be validating that any set of regions added to the builder have the correct axis count.

I also believe that if we know the number of axes ahead of time there may be possible improvements in the implementation, but I may be confusing axis count and region count.

> If we don't already we should probably validate that the regions match this count since that's possible to check with access to only self?

_Originally posted by @rsheeter in https://github.com/googlefonts/fontations/pull/742#discussion_r1428604171_