Some improvements for the plugin

1. Make the completions work with the custom install location. The sdkman install location is hardcoded in the original completion file.
2. Replace the tr command with fish shell builtin command

completions/sdk.fish

@@ -60,21 +60,21 @@ end
 # # # # # #
 
 function __fish_sdkman_candidates
-    cat "$HOME"/.sdkman/var/candidates | tr ',' '\n'
+    cat "$SDKMAN_DIR"/var/candidates | string replace -a -r ',' '\n'
 end
 
 function __fish_sdkman_candidates_with_versions
     set regexpHome (string replace -a '/' '\\/' "$HOME/")
 
-    find "$HOME"/.sdkman/candidates/ -mindepth 2 -maxdepth 2 -name '*current' \
+    find "$SDKMAN_DIR"/candidates/ -mindepth 2 -maxdepth 2 -name '*current' \
     | awk -F '/' '{ print $(NF-1) }' \
     | sort -u
 end
 
 function __fish_sdkman_installed_versions
     set cmd (commandline -opc)
-    if [ -d "$HOME"/.sdkman/candidates/$cmd[3]/current ]
-        ls -v1 "$HOME"/.sdkman/candidates/$cmd[3] | grep -v current
+    if [ -d "$SDKMAN_DIR"/candidates/$cmd[3]/current ]
+        ls -v1 "$SDKMAN_DIR"/candidates/$cmd[3] | grep -v current
     end
 end