feat: Add xdg_basedir module to respect XDG Basedir spec even on macOS

[poliorcetics]

This is mostly useful for CLI tools and people who want to have their config files in the same
place on both macOS and Linux, making it much easier to save them in a VCS repo.

[poliorcetics]

Follow up works will need to be done on https://github.com/dirs-dev/directories-rs to support it nicely.
Since the changes here are not API breaking, a minor version can be released and used in directories

[soc]

Sorry, as mentioned in the original issue, I don't think this is a good idea.

The dirs crates are intended to follow platform rules, not invent their own conventions.

Not following the OS rules due to being a CLI tool sounds like a reason just as bad as the dozen other excuses people come up with to litter $HOME with their application folders. (Which is the reason we are in this mess in the first place.)

Not to mention the additional confusion and support requests this will cause.

If you want XDG rules use Linux, if you want Apple rules use a Mac.

[poliorcetics]

The slew of requests asking for this on macOS seems to indicate a need exists for this feature, just as the number of Rust CLI tools that entirely bypass this crate and use another way to get support for the XDG spec on macOS

[soc]

entirely bypass this crate and use another way to get support for the XDG spec on macOS

That's what I generally recommend. It doesn't make sense to use a crate that follows the rules of the operating system, while trying to avoid the rules.