Save and restore explorer branches (close to bookmarks)

[echasnovski]

Contributing guidelines

• I have read CONTRIBUTING.md
• I have read CODE_OF_CONDUCT.md

Module(s)

mini.files

Description

Consider adding functions and mappings for saving and restoring explorer branches for quick navigation to known destinations.

Common problem during file system manipulations is the need to hhhhjjjllll in order to navigate back and forth between two directories. This can be solved by "save current branch under identifier x" and "restore branch under identifier x" actions. Similar to how built-in marks work inside and across files.

Saving branches (like { '/home/user/.config', '/home/user/.config/nvim' }) instead of single destination directory (like '/home/user/.config/nvim') seems to both provide more flexibility and be more aligned with internal implementation.

This can also solve the problem of #572 by saving predefined set of branches inside autocommand for MiniFilesExplorerOpen event.

Notes:

• Usual issue of "it can need a lot of new lines (code + doc + tests)" is very valid here. So it can still be deemed too much.
• Default mapping keys maybe can be m and ' to mimic built-in marks.
• I don't think having get_branch() and set_branch() are necessary for this.