LibWeb: Clear exceptions in each Document::run_javascript() call

We don't want to carry over exceptions across multiple
Document::run_javascript() calls as Interpreter::run() and every of its
exception checks will get confused - in this case there would be an
exception, but not because a certain action failed.

Real-life example:

<script>var a = {}; a.test()</script>
<script>alert("It worked!")</script>

The above HTML will invoke Document::run_javascript() twice, the first
call will result in a TypeError, which is still stored during the second
call. The interpreter will eventually call the following functions (in
order) for the alert() invocation:

- Identifier::execute()
- Interpreter::get_variable()
- Object::get() (on the global object)

That last Object::get() call has an exception check which is triggered
as we still carry around the exception from earlier - and eventually
returns an empty value.

Long story short, the second script will wrongly fail with
"ReferenceError, 'alert' is not defined".

Fixes #3091.

Libraries/LibWeb/DOM/Document.cpp

@@ -436,7 +436,11 @@ JS::Value Document::run_javascript(const StringView& source)
         parser.print_errors();
         return JS::js_undefined();
     }
-    return document().interpreter().run(document().interpreter().global_object(), *program);
+    auto& interpreter = document().interpreter();
+    auto result = interpreter.run(interpreter.global_object(), *program);
+    if (interpreter.exception())
+        interpreter.clear_exception();
+    return result;
 }
 
 NonnullRefPtr<Element> Document::create_element(const String& tag_name)