[web/debugger.js] Enable the ESLint `no-var rule

These changes were made *mostly* automatically, using `gulp lint --fix`, with the following manual changes:

```diff
diff --git a/web/debugger.js b/web/debugger.js
index 1cda4066e..6f8b4a9f0 100644
--- a/web/debugger.js
+++ b/web/debugger.js
@@ -264,10 +264,9 @@ const Stepper = (function StepperClosure() {
     }
     if ("length" in args) {
       // array
-      let simpleArgs = [],
-        i,
-        ii;
-      const MAX_ITEMS = 10;
+      const MAX_ITEMS = 10,
+        simpleArgs = [];
+      let i, ii;
       for (i = 0, ii = Math.min(MAX_ITEMS, args.length); i < ii; i++) {
         simpleArgs.push(simplifyArgs(args[i]));
       }
@@ -511,11 +510,8 @@ window.PDFBug = (function PDFBugClosure() {
   return {
     tools: [FontInspector, StepperManager, Stats],
     enable(ids) {
-      let all = false,
-        tools = this.tools;
-      if (ids.length === 1 && ids[0] === "all") {
-        all = true;
-      }
+      const all = ids.length === 1 && ids[0] === "all";
+      const tools = this.tools;
       for (let i = 0; i < tools.length; ++i) {
         const tool = tools[i];
         if (all || ids.includes(tool.id)) {
```