Add ability to lookup parameter name in urlskip=

Relate case:
uBlockOrigin/uBlock-issues#3206 (comment)

Newly supported step: `&i`, meant to lookup a parameter's name at
position `i` (1-based). The parameter name will be used as the
URL (whereas `?` is meant to lookup a parameter's value).

Author: gorhill

src/js/static-filtering-parser.js

@@ -1531,7 +1531,7 @@ export class AstFilterParser {
                 break;
             }
             const value = this.getNetOptionValue(NODE_TYPE_NET_OPTION_NAME_URLSKIP);
-            if ( value.startsWith('?') === false || value.length < 2 ) {
+            if ( value.length < 2 ) {
                 this.astError = AST_ERROR_OPTION_BADVALUE;
                 realBad = true;
             }

src/js/static-net-filtering.js

@@ -5411,8 +5411,9 @@ function urlSkip(urlin, steps) {
     try {
         let urlout;
         for ( const step of steps ) {
+            const c0 = step.charCodeAt(0);
             // Extract from URL parameter
-            if ( step.startsWith('?') ) {
+            if ( c0 === 0x3F ) { /* ? */
                 urlout = (new URL(urlin)).searchParams.get(step.slice(1));
                 if ( urlout === null ) { return; }
                 if ( urlout.includes(' ') ) {
@@ -5421,6 +5422,16 @@ function urlSkip(urlin, steps) {
                 urlin = urlout;
                 continue;
             }
+            // Extract from URL parameter name at position i
+            if ( c0 === 0x26 ) { /* & */
+                const i = (parseInt(step.slice(1)) || 0) - 1;
+                if ( i < 0 ) { return; }
+                const url = new URL(urlin);
+                if ( i >= url.searchParams.size ) { return; }
+                const params = Array.from(url.searchParams.keys());
+                urlin = urlout = decodeURIComponent(params[i]);
+                continue;
+            }
             // Enforce https
             if ( step === '+https' ) {
                 const s = urlin.replace(/^https?:\/\//, '');