Use XDG base directories and Known Folders on Windows (#1809)

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`

---------

Co-authored-by: Florin Ungur <florin@florinungur.com>

docs/modules/release-notes/pages/0.33.adoc

@@ -13,7 +13,43 @@ include::partial$intro.adoc[]
 
 == Noteworthy [small]#🎶#
 
-=== XXX
+=== CLI Changes
+
+==== Default file locations
+
+For new setups, the CLI no longer stores anything under `~/.pkl` (https://github.com/apple/pkl/pull/1809[#1809]).
+
+It uses XDG-style locations on Unix and Known Folder locations on Windows:
+
+[cols="1,2,2,2",options="header"]
+|===
+| Concern | Unix (Linux/macOS) | Windows | Legacy fallback
+
+| Package cache
+| `~/.cache/pkl`
+| `$LOCALAPPDATA/pkl/Cache`
+| none
+
+| Settings file
+| `~/.config/pkl/settings.pkl`
+| `$APPDATA/pkl/settings.pkl`
+| `~/.pkl/settings.pkl`
+
+| CA certificates
+| `~/.config/pkl/cacerts`
+| `$APPDATA/pkl/cacerts`
+| `~/.pkl/cacerts`
+
+| REPL history
+| `~/.local/state/pkl/repl-history`
+| `$LOCALAPPDATA/pkl/repl-history`
+| none
+|===
+
+On every OS, these locations can be overridden with XDG-style env vars.
+For example, setting `XDG_CACHE_HOME` will configure the cache directory.
+
+Note that the existing `~/.pkl/cache` directory is ignored, so Pkl will download packages to populate its cache if configured to do so.
 
 == Breaking Changes [small]#💔#