Improve href-sanitizer scriptlet

Support ability to recursively unwrap destination URL. Example:

    ...##+js(href-sanitizer, a.clickTracker, ?r?u)

Related discussion:
uBlockOrigin/uBlock-discussions#775 (comment)

assets/resources/scriptlets.js

@@ -3503,17 +3503,26 @@ function hrefSanitizer(
         }
         return '';
     };
+    const extractParam = (href, source) => {
+        if ( Boolean(source) === false ) { return href; }
+        const recursive = source.includes('?', 1);
+        const end = recursive ? source.indexOf('?', 1) : source.length;
+        try {
+            const url = new URL(href, document.location);
+            const value = url.searchParams.get(source.slice(1, end));
+            if ( value === null ) { return href }
+            if ( recursive ) { return extractParam(value, source.slice(end)); }
+            return value;
+        } catch(x) {
+        }
+        return href;
+    };
     const extractText = (elem, source) => {
         if ( /^\[.*\]$/.test(source) ) {
             return elem.getAttribute(source.slice(1,-1).trim()) || '';
         }
         if ( source.startsWith('?') ) {
-            try {
-                const url = new URL(elem.href, document.location);
-                return url.searchParams.get(source.slice(1)) || '';
-            } catch(x) {
-            }
-            return '';
+            return extractParam(elem.href, source);
         }
         if ( source === 'text' ) {
             return elem.textContent