Add alternate array index style, with setting

src/Editor/Editor.php

@@ -806,7 +806,7 @@ class Editor
                 'quotedKeys' => (Settings::$editorQuotedKeys?$mark:$clear)."Quoted keys",
                 'tailLine' => (Settings::$tailLine?$mark:$clear)."Show tail line",
                 'indentationGuides' => (Settings::$indentationGuides?$mark:$clear)."Show indentation guides",
-                //'done' => "  Done"
+                'highlightIndices' => (Settings::$highlightIndices?$mark:$clear)."Highlight array indices",
             ];
             $sel = (new Menu($this->term, $items, "Settings"))
                 ->display(0, 0, 40, 0, $sel);
@@ -827,6 +827,9 @@ class Editor
                 case 'indentationGuides':
                     Settings::$indentationGuides = !Settings::$indentationGuides;
                     break;
+                case 'highlightIndices':
+                    Settings::$highlightIndices = !Settings::$highlightIndices;
+                    break;
             }
         } while ($sel);
         $this->redrawEditor();