install.sh: Honor XDG_DATA_HOME

[why]
When a user sets the XDG_DATA_HOME environment variable the default of
$HOME/.local/share should not be used.

https://wiki.archlinux.org/title/XDG_Base_Directory

[how]
Enable XDG_DATA_HOME to override the default user data directory on Unix
and MacOS platforms.

There is some controversy if XDG_DATA_HOME should be honored on MacOS,
see for example platformdirs (link below). But me also working on Linux
and MacOS (and Windows) in parallel, I can not quite follow the
downsides and believe we should allow XDG_DATA_HOME also for MacOS.

Related: https://github.com/platformdirs/platformdirs/issues/4
Fixes: #1324

Suggested-by: Anthony Foglia <@afoglia>
Signed-off-by: Fini Jastrow <ulf.fini.jastrow@desy.de>

install.sh

@@ -1,6 +1,6 @@
 #!/usr/bin/env bash
 # Install Nerd Fonts
-__ScriptVersion="0.6"
+__ScriptVersion="0.7"
 
 # Default values for option variables:
 quiet=false
@@ -197,15 +197,20 @@ done
 # Get target root directory
 if [[ $(uname) == 'Darwin' ]]; then
   # MacOS
-  sys_font_dir="/Library/Fonts"
-  usr_font_dir="$HOME/Library/Fonts"
+  sys_share_dir="/Library"
+  usr_share_dir="$HOME/Library"
+  font_subdir="Fonts"
 else
   # Linux
-  sys_font_dir="/usr/local/share/fonts"
-  usr_font_dir="$HOME/.local/share/fonts"
+  sys_share_dir="/usr/local/share"
+  usr_share_dir="$HOME/.local/share"
+  font_subdir="fonts"
 fi
-sys_font_dir="${sys_font_dir}/NerdFonts"
-usr_font_dir="${usr_font_dir}/NerdFonts"
+if [ -n "${XDG_DATA_HOME}" ]; then
+  usr_share_dir="${XDG_DATA_HOME}"
+fi
+sys_font_dir="${sys_share_dir}/${font_subdir}/NerdFonts"
+usr_font_dir="${usr_share_dir}/${font_subdir}/NerdFonts"
 
 if [[ "system" == "$installpath" ]]; then
   font_dir="${sys_font_dir}"