treewide: prefer XDG variables over dot directories

Currently, dot directories and XDG base directories are used
inconsistently in the Home Manager option declarations. This creates
ambiguity for the user as to where the location of the file should be
albeit this is rarely encountered in practice as it is sufficient to
read upstream documentation. The rationale is to make declarations
consistent and make a clear distinction between hardcoded and modular
specifications.

References to ~/.config in relevant nixpkgs modules were untouched as
the location is hardcoded upstream[1]. Furthermore, modules of
programs which do not follow XDG specifications were also untouched.

Generalization of tilde(~) expansions to $HOME were also considered,
however there isn't sufficient rationale despite the use of $HOME
being more universal. The expansion is standardized in POSIX[2] and is
essentially portable across all shells, thus there is no pragmatic
value to introducing the change.

[1] https://github.com/nixos/nixpkgs/blob/master/pkgs/top-level/impure.nix
[2] https://pubs.opengroup.org/onlinepubs/009695399/utilities/xcu_chap02.html#tag_02_06_01

modules/programs/lazygit.nix

@@ -32,7 +32,7 @@ in {
       '';
       description = ''
         Configuration written to
-        <filename>~/.config/lazygit/config.yml</filename> on Linux
+        <filename>$XDG_CONFIG_HOME/lazygit/config.yml</filename> on Linux
         or <filename>~/Library/Application Support/lazygit/config.yml</filename> on Darwin. See
         <link xlink:href="https://github.com/jesseduffield/lazygit/blob/master/docs/Config.md"/>
         for supported values.