Support `NSCollectionView` sections

[helje5]

Checklist

• Reviewed the README and documents.
• Searched existing issues for ensure not duplicated.

Description

The NSCollectionView reload extension method doesn't support sections.

Motivation and Context

Would be nice to have sections, just like on iOS :-)

Proposed Solution

Unlike #93 (NSTableView section support) this one should be an almost exact copy of the UICollectionView.reload implementation? AFAIK the NSCollectionView essentially works the same, at least since ~10.12. (The main difference is that NSCollectionView uses VCs instead of plain views as items, but that doesn't matter here).
deleteSections/insertSections etc. are all available.

[ra1028]

Thanks @helje5 .
Sorry for my late reply.
I agree with that, but I'm not familiar with AppKit.
Feel free to make a pull-request!