Move the debuggerSrc-parameter into the AppOptions

Having this parameter among a list of DOM-elements seems slightly strange now, however this is very old code hence the explanation for why this was done is for historical reasons (as is often the case).
Hence we can simply move this into `AppOptions` instead, which seems more appropriate overall.

web/app_options.js

@@ -105,6 +105,11 @@ const defaultOptions = {
     value: 0,
     kind: OptionKind.VIEWER + OptionKind.PREFERENCE,
   },
+  debuggerSrc: {
+    /** @type {string} */
+    value: "./debugger.mjs",
+    kind: OptionKind.VIEWER,
+  },
   defaultZoomDelay: {
     /** @type {number} */
     value: 400,