Packer.nvim: [Feature request] Support package snapshot

[weihanglo]

Sometimes upgrading packages may results in conflicts between packages. By having a snapshot of all current packages with exact commits, we can restore from the snapshot without much labor.

Reference: vim-plug supports snapshot.

[wbthomason]

Thanks for the idea and reference! I'd be happy to help guide a PR implementing this (it should be pretty straightforward), if you or anyone else is interested.

[weihanglo]

Yes, I can help! It would be awesome to have some instruction.