fix: ignore relative XDG base directory paths

[lakshayxi]

Fixes #539.

The XDG Base Directory Specification requires XDG directory environment variables to contain absolute paths. Relative values must be ignored.

This change:

• validates XDG base directory variables before using them;
• ignores relative values for XDG_DATA_HOME, XDG_CONFIG_HOME, XDG_CACHE_HOME, XDG_STATE_HOME, and XDG_RUNTIME_DIR;
• filters relative entries from XDG_DATA_DIRS and XDG_CONFIG_DIRS while preserving valid absolute entries;
• applies the same validation when locating user-dirs.dirs;
• preserves the existing platform-specific fallback behavior.

Tests cover Unix and macOS behavior, including mixed valid/relative directory lists and derived XDG data-directory consumers.