paths: XDG_CONFIG_HOME compliance on non-linux platforms

[chawyehsu]

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This resolves https://github.com/zed-industries/zed/discussions/27338

Release Notes:

• The XDG Base Directory convention is platform-agnostic — meaning it can be used on non-linux platforms when env vars are explicitly configured.

[zeenix]

@chawyehsu I've a tiny crate for this, if it helps: https://crates.io/crates/xdg-home

[mikayla-maki]

I think rather than having this rewrite, we'd prefer it if this kind of change got upstreamed to dirs, or a fork of dirs. We already have enough complexity in our config path selection, and I'd like to keep it down if we can :)