- Linting (ESLint): Update polyfills to new compat rules of

eslint-config-ash-nazg and adhere to new rules (prefer `for-of`
  (or array methods) to `for`, catch preferred `includes` to `indexOf`);
  avoid `no-zero-fractions` rule for now
- npm: Update devDeps (removing one unneeded)

screencasts/svgopen2010/script.js

@@ -180,7 +180,7 @@ const SlideShow = function (slides) {
 SlideShow.prototype = {
   _slides: [],
   _update (dontPush) {
-    document.querySelector('#presentation-counter').innerText = this.current;
+    document.querySelector('#presentation-counter').textContent = this.current;
     if (history.pushState) {
       if (!dontPush) {
         history.pushState(this.current, 'Slide ' + this.current, '#slide' + this.current);