Add trusted-click-element scriptlet

Implemented as per AdGuard API documentation:
https://github.com/AdguardTeam/Scriptlets/blob/master/wiki/about-trusted-scriptlets.md#trusted-click-element

The current implementation in uBO does not support the `extraMatch`
argument. If the extraMatch argument is not empty, the scriptlet
will abort and do nothing.

As for the rest, it complies with the documentation.

assets/resources/scriptlets.js

@@ -224,9 +224,9 @@ function runAt(fn, when) {
 
 builtinScriptlets.push({
     name: 'run-at-html-element.fn',
-    fn: runAtHtmlElement,
+    fn: runAtHtmlElementFn,
 });
-function runAtHtmlElement(fn) {
+function runAtHtmlElementFn(fn) {
     if ( document.documentElement ) {
         fn();
         return;
@@ -1160,7 +1160,7 @@ builtinScriptlets.push({
 // Issues to mind before changing anything:
 //  https://github.com/uBlockOrigin/uBlock-issues/issues/2154
 function abortCurrentScript(...args) {
-    runAtHtmlElement(( ) => {
+    runAtHtmlElementFn(( ) => {
         abortCurrentScriptCore(...args);
     });
 }
@@ -3882,4 +3882,113 @@ function trustedReplaceXhrResponse(
     };
 }
 
+/*******************************************************************************
+ * 
+ * trusted-click-element.js
+ * 
+ * Reference API:
+ * https://github.com/AdguardTeam/Scriptlets/blob/master/src/scriptlets/trusted-click-element.js
+ * 
+ **/
+
+builtinScriptlets.push({
+    name: 'trusted-click-element.js',
+    requiresTrust: true,
+    fn: trustedClickElement,
+    world: 'ISOLATED',
+    dependencies: [
+        'run-at-html-element.fn',
+        'safe-self.fn',
+    ],
+});
+function trustedClickElement(
+    selectors = '',
+    extraMatch = '', // not yet supported
+    delay = ''
+) {
+    if ( extraMatch !== '' ) { return; }
+
+    const selectorList = selectors.split(/\s*,\s*/)
+        .filter(s => {
+            try {
+                void document.querySelector(s);
+            } catch(_) {
+                return false;
+            }
+            return true;
+        });
+    if ( selectorList.length === 0 ) { return; }
+
+    const clickDelay = parseInt(delay, 10) || 1;
+    const t0 = Date.now();
+    const tbye = t0 + 10000;
+    let tnext = t0;
+
+    const terminate = ( ) => {
+        selectorList.length = 0;
+        next.stop();
+        observe.stop();
+    };
+
+    const next = notFound => {
+        if ( selectorList.length === 0 ) { return terminate(); }
+        const tnow = Date.now();
+        if ( tnow >= tbye ) { return terminate(); }
+        if ( notFound ) { observe(); }
+        const delay = Math.max(notFound ? tbye - tnow : tnext - tnow, 1);
+        next.timer = setTimeout(( ) => {
+            next.timer = undefined;
+            process();
+        }, delay);
+    };
+    next.stop = ( ) => {
+        if ( next.timer === undefined ) { return; }
+        clearTimeout(next.timer);
+        next.timer = undefined;
+    };
+
+    const observe = ( ) => {
+        if ( observe.observer !== undefined ) { return; }
+        observe.observer = new MutationObserver(( ) => {
+            if ( observe.timer !== undefined ) { return; }
+            observe.timer = setTimeout(( ) => {
+                observe.timer = undefined;
+                process();
+            }, 20);
+        });
+        observe.observer.observe(document, {
+            attributes: true,
+            childList: true,
+            subtree: true,
+        });
+    };
+    observe.stop = ( ) => {
+        if ( observe.timer !== undefined ) {
+            clearTimeout(observe.timer);
+            observe.timer = undefined;
+        }
+        if ( observe.observer ) {
+            observe.observer.disconnect();
+            observe.observer = undefined;
+        }
+    };
+
+    const process = ( ) => {
+        next.stop();
+        if ( Date.now() < tnext ) { return next(); }
+        const selector = selectorList.shift();
+        if ( selector === undefined ) { return terminate(); }
+        const elem = document.querySelector(selector);
+        if ( elem === null ) {
+            selectorList.unshift(selector);
+            return next(true);
+        }
+        elem.click();
+        tnext += clickDelay;
+        next();
+    };
+
+    runAtHtmlElementFn(process);
+}
+
 /******************************************************************************/