Re-implement working dev-sandbox/watch-dev-sandbox gulp-tasks

Compared to the, previously removed, `sandbox`/`watch-sandbox` gulp-tasks, these ones should work even when run against an non-existent/empty `build`-folder.

Also, to ensure that the development viewer actually works out-of-the-box, `gulp server` will now also include `gulp watch-dev-sandbox` to remove the need to *manually* invoke the build-tasks.

Finally, this patch also removes the `web/devcom.js` file since it shouldn't actually be needed, assuming that the "sandbox"-loading code in the `web/genericcom.js` file is actually *correctly* implemented.

web/app_options.js

@@ -223,14 +223,6 @@ const defaultOptions = {
     value: false,
     kind: OptionKind.API,
   },
-  scriptingSrc: {
-    /** @type {string} */
-    value:
-      typeof PDFJSDev === "undefined" || !PDFJSDev.test("PRODUCTION")
-        ? "../build/generic/build/pdf.sandbox.js"
-        : "../build/pdf.sandbox.js",
-    kind: OptionKind.VIEWER,
-  },
   verbosity: {
     /** @type {number} */
     value: 1,
@@ -265,6 +257,14 @@ if (
     value: typeof navigator !== "undefined" ? navigator.language : "en-US",
     kind: OptionKind.VIEWER,
   };
+  defaultOptions.sandboxBundleSrc = {
+    /** @type {string} */
+    value:
+      typeof PDFJSDev === "undefined" || !PDFJSDev.test("PRODUCTION")
+        ? "../build/dev-sandbox/pdf.sandbox.js"
+        : "../build/pdf.sandbox.js",
+    kind: OptionKind.VIEWER,
+  };
 }
 
 const userOptions = Object.create(null);