This changes the `join` method in `List`/`Set`/`Listing` to
call the overridden `toString()` method.
This is a breaking change, but is considered a bugfix.
This changes logic that previously read/wrote from `~/.pkl` to use
XDG base directories (all OSes), and Known Folders locations on Windows.
For example, Pkl will look for `settings.pkl` in:
1. `$XDG_CONFIG_HOME/pkl/settings.pkl`
2. `%APPDATA/pkl/settings.pkl`
3. `~/.pkl/settings.pkl`
4. Path pkl/settings/pkl within `$XDG_CONFIG_DIRS`
5. `/etc/xdg/pkl/settings.pkl`
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Co-authored-by: Florin Ungur <florin@florinungur.com>
Jen Basch
