Enter polyfills.js

The `String.replaceAll` polyfill was borrowed (at 0% annual intereset
rate) from https://github.com/kiwix/kiwix-js/pull/1190/files.

static/skin/polyfills.js

@@ -0,0 +1,21 @@
+// A few browsers do not support the use of String.prototype.replaceAll method.
+// Hence we define it once we verify that it isn't supported. For documentation
+// see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/replaceAll
+if (!String.prototype.replaceAll) {
+    String.prototype.replaceAll = function (pattern, replacement) {
+        // verify parameter: It must either be a string or a RegExp with a global flag.
+        if (typeof pattern[Symbol.replace] === 'function') {
+            // the pattern is a RegExp check for the presence of g flag.
+            if (pattern.global) {
+                return this.replace(pattern, replacement);
+            } else {
+                throw new TypeError('Global flag for regular expressions')
+            }
+        }
+        // the pattern is not a RegExp, hence it must be a string.
+        if (typeof pattern !== 'string') {
+            throw new TypeError('pattern must either be a string or a RegExp with a global (g) flag.')
+        }
+        return this.replace(new RegExp(pattern, 'g'), replacement);
+    }
+}