fix backspace in editor

Author: pepc84

editor/index.html

@@ -152,6 +152,7 @@
     'Cmd-Enter': runSketch,
     'Tab': cm => cm.execCommand('indentMore'),
     'Shift-Tab': cm => cm.execCommand('indentLess'),
+    'Backspace': cm => cm.execCommand('delCharBefore'),
   },
 });