Introduce a $kak_config env var containing the Kakoune user config dir

Makes it easier for users who want to locate their kakrc file, and
does not require to go through shell expansion to get it as
"${XDG_CONFIG_DIR:-${HOME}/.config}/kak"

Fixes #1740

share/kak/kakrc

@@ -1,7 +1,6 @@
 def -params 1 -docstring "colorscheme <name>: enable named colorscheme" \
   -shell-candidates %{
-      localconfdir=${XDG_CONFIG_HOME:-${HOME}/.config}/kak
-      for col in ${kak_runtime}/colors/*.kak ${localconfdir}/colors/*.kak; do
+      for col in ${kak_runtime}/colors/*.kak ${kak_config}/colors/*.kak; do
           candidate=$(basename "${col}" .kak)
           case ${candidate} in
              \**) ;;
@@ -10,10 +9,9 @@ def -params 1 -docstring "colorscheme <name>: enable named colorscheme" \
       done
   } \
   colorscheme %{ %sh{
-    localconfdir=${XDG_CONFIG_HOME:-${HOME}/.config}/kak
     if [ $# -eq 1 ]; then
-        if [ -f "${localconfdir}/colors/$1.kak" ];then
-            echo "source '${localconfdir}/colors/$1.kak'"
+        if [ -f "${kak_config}/colors/$1.kak" ];then
+            echo "source '${kak_config}/colors/$1.kak'"
         else
             echo "source '${kak_runtime}/colors/$1.kak'"
         fi
@@ -28,12 +26,10 @@ def -params 1 -docstring "colorscheme <name>: enable named colorscheme" \
             -exec printf 'try %%{ source "%s" } catch %%{ echo -debug Autoload: could not load "%s" }\n' '{}' '{}' \;
     }
 
-    localconfdir=${XDG_CONFIG_HOME:-${HOME}/.config}/kak
-
     echo "colorscheme default"
 
-    if [ -d "${localconfdir}/autoload" ]; then
-        autoload_directory ${localconfdir}/autoload
+    if [ -d "${kak_config}/autoload" ]; then
+        autoload_directory ${kak_config}/autoload
     elif [ -d "${kak_runtime}/autoload" ]; then
         autoload_directory ${kak_runtime}/autoload
     fi
@@ -42,7 +38,7 @@ def -params 1 -docstring "colorscheme <name>: enable named colorscheme" \
         echo "source '${kak_runtime}/kakrc.local'"
     fi
 
-    if [ -f "${localconfdir}/kakrc" ]; then
-        echo "source '${localconfdir}/kakrc'"
+    if [ -f "${kak_config}/kakrc" ]; then
+        echo "source '${kak_config}/kakrc'"
     fi
 }

src/main.cc

@@ -68,6 +68,14 @@ String runtime_directory()
     return "/usr/share/kak";
 }
 
+String config_directory()
+{
+    StringView config_home = getenv("XDG_CONFIG_HOME");
+    if (config_home.empty())
+        return format("{}/.config/kak", getenv("HOME"));
+    return format("{}/kak", config_home);
+}
+
 void register_env_vars()
 {
     static const struct {
@@ -112,6 +120,10 @@ void register_env_vars()
             "runtime", false,
             [](StringView name, const Context& context)
             { return runtime_directory(); }
+        }, {
+            "config", false,
+            [](StringView name, const Context& context)
+            { return config_directory(); }
         }, {
             "opt_", true,
             [](StringView name, const Context& context)