Add a keybinding for collapsing the directory the cursor is in - #6066

I've read CONTRIBUTING.md, and I know this project doesn't accept pull requests.
I'm opening this one anyway, so let me be upfront about why.

I wanted this feature badly enough to build it for my own daily use.

If it isn't to your taste, please just close it — no explanation needed, and no hard feelings. I'll keep using it in my fork either way. But I do hope that something like this makes it into lazygit one day.

PR Description

Working in a deeply nested directory and wanting to fold it away means first walking the cursor up to the directory's own line and only then collapsing it. This binds that to a single key, leaving the cursor on the directory that was just collapsed so it can be repeated to fold up the tree one level at a time.

The containing directory is found by scanning upwards for the first item rendered at a smaller visual depth, rather than by taking the parent of the selected item's path. Paths don't survive either of the tree's display transformations: a chain of single-child directories is compressed into one node whose path names the deepest of them, so the path's parent is often a node that doesn't exist, and with showRootItemInFileTree disabled the internal paths carry no "./" prefix. Visual depth is exactly what's on screen, so both fall out for free.

Please check if the PR fulfills these requirements

• Cheatsheets are up-to-date (run go generate ./...)
• Code has been formatted (see here)
• Tests have been added/updated (see here for the integration test guide)
• Text is internationalised (see here)
• If a new UserConfig entry was added, make sure it can be hot-reloaded (see here)
• Docs have been updated if necessary
• You've read through your own file changes for silly mistakes etc