Support XDG_CONFIG_HOME (#195)

* Support XDG_CONFIG_HOME

Used approach from:
https://unix.stackexchange.com/questions/537529/how-do-i-get-xdg-config-home-from-a-shell-script-command-line
https://gitlab.freedesktop.org/xdg/xdg-user-dirs/-/blob/97ff189affdbf0d33c0f8e91d3fd5608f425ef96/xdg-user-dir

* Document XDG_CONFIG_HOME in README also

Author: axelson

README.md

@@ -137,8 +137,8 @@ If you're packaging these archives in an IDE plugin, make sure to build using th
 Because ElixirLS may get launched from an IDE that itself got launched from a graphical shell, the environment may not
 be complete enough to run or even find the correct Elixir/OTP version. The wrapper scripts try to configure `asdf-vm`
 if available, but that may not be what you want or need. Therefore, prior to executing Elixir, the script will source
-`~/.config/elixir_ls/setup.sh`, if available. The environment variable `ELS_MODE` is set to either `debugger` or
-`language_server` to help you decide what to do inside the script, if needed.
+`$XDG_CONFIG_HOME/elixir_ls/setup.sh` (e.g. `~/.config/elixir_ls/setup.sh`), if available. The environment variable
+`ELS_MODE` is set to either `debugger` or `language_server` to help you decide what to do inside the script, if needed.
 
 ## Acknowledgements and related projects
 

apps/elixir_ls_utils/priv/launch.sh

@@ -27,8 +27,8 @@ then
       fi
       ;;
     *)
-      # We have an arg2, so we got relaunched. Therefore, e're running in
-      # a shell that can asdf-vm.
+      # We have an arg2, so we got relaunched. Therefore, we're running in a
+      # shell that supports asdf-vm.
       .  "${asdf_vm}"
       ;;
   esac
@@ -39,7 +39,7 @@ fi
 # give them the chance here. ELS_MODE will be set for
 # the really complex stuff. Use an XDG compliant path.
 
-els_setup="${HOME}/.config/elixir_ls/setup.sh"
+els_setup="${XDG_CONFIG_HOME:-~/.config}/elixir_ls/setup.sh"
 if test -f "${els_setup}"
 then
   .  "${els_setup}"