A setting to follow XDG Base Directory standard

XDG Base Directory is a standard for locations for storing various
files. Nix has a few files which seem to fit in the standard, but
currently use a custom location directly in the user's ~, polluting
it:

- ~/.nix-profile
- ~/.nix-defexpr
- ~/.nix-channels

This commit adds a config option (use-xdg-base-directories) to follow
the XDG spec and instead use the following locations:

- $XDG_STATE_HOME/nix/profile
- $XDG_STATE_HOME/nix/defexpr
- $XDG_STATE_HOME/nix/channels

If $XDG_STATE_HOME is not set, it is assumed to be ~/.local/state.

Co-authored-by: Théophane Hufschmitt <7226587+thufschmitt@users.noreply.github.com>
Co-authored-by: Tim Fenney <kodekata@gmail.com>
Co-authored-by: pasqui23 <pasqui23@users.noreply.github.com>
Co-authored-by: Artturin <Artturin@artturin.com>
Co-authored-by: John Ericson <Ericson2314@Yahoo.com>

src/libstore/globals.hh

@@ -975,6 +975,27 @@ public:
           resolves to a different location from that of the build machine. You
           can enable this setting if you are sure you're not going to do that.
         )"};
+
+    Setting<bool> useXDGBaseDirectories{
+        this, false, "use-xdg-base-directories",
+        R"(
+          If set to `true`, Nix will conform to the [XDG Base Directory Specification] for files in `$HOME`.
+          The environment variables used to implement this are documented in the [Environment Variables section](@docroot@/installation/env-variables.md).
+
+          [XDG Base Directory Specification]: https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html
+
+          > **Warning**
+          > This changes the location of some well-known symlinks that Nix creates, which might break tools that rely on the old, non-XDG-conformant locations.
+
+          In particular, the following locations change:
+
+          | Old               | New                            |
+          |-------------------|--------------------------------|
+          | `~/.nix-profile`  | `$XDG_STATE_HOME/nix/profile`  |
+          | `~/.nix-defexpr`  | `$XDG_STATE_HOME/nix/defexpr`  |
+          | `~/.nix-channels` | `$XDG_STATE_HOME/nix/channels` |
+        )"
+    };
 };