feat: adopt XDG Base Directory standard for config paths

[chloe41427]

Summary

Adopt the XDG Base Directory Specification for config file locations in the native CLI.

Motivation

Currently SDKMAN! stores its config at ~/.sdkman/etc/config. The XDG standard defines $XDG_CONFIG_HOME (defaulting to ~/.config) as the canonical location for user config files. Several community members have requested support for this — see sdkman-cli#1069 for prior art.

As part of the bash CLI rewrite, we plan to adopt XDG standards holistically. The native CLI should follow suit.

Requirements

• Read user config from $XDG_CONFIG_HOME/sdkman/config (defaulting to ~/.config/sdkman/config)
• Respect $XDG_DATA_HOME for data files (defaulting to ~/.local/share)
• Maintain backwards compatibility with existing ~/.sdkman layout during a transition period
• Follow precedence rules: XDG config overrides legacy config if both exist

References

• XDG Base Directory Specification: https://specifications.freedesktop.org/basedir-spec/latest/
• Prior community request: Allow ~/.config/sdkman/config to override ~/.sdkman/etc/config sdkman-cli#1069

[chloe41427]

Note on macOS / Darwin

The XDG Base Directory Specification is a freedesktop.org standard targeting Linux desktop environments. macOS has its own conventions:

• Config: ~/Library/Preferences/
• App data: ~/Library/Application Support/
• Cache: ~/Library/Caches/

For CLI tools, the pragmatic approach is to:

• Linux: respect $XDG_CONFIG_HOME (defaulting to ~/.config/sdkman/config)
• macOS: use ~/Library/Application Support/sdkman/config by default, but honour $XDG_CONFIG_HOME if explicitly set by the user

This is the behaviour provided out of the box by the Rust dirs crate, which follows platform conventions while still respecting XDG environment variables on macOS when set. Recommended to lean on that rather than rolling our own path resolution.