mini.files: bookmark persistence across nvim sessions

[proofer]

I would like a way to have mini.files bookmarks persist across nvim sessions. This would require on-disk storage, so I think it would have to be a configuration option.

[echasnovski]

The suggested way to do that is present in examples. That is, create an autocommand for MiniFilesExplorerOpen event and set custom bookmarks inside it.