Everywhere: Rename ASSERT => VERIFY

(...and ASSERT_NOT_REACHED => VERIFY_NOT_REACHED)

Since all of these checks are done in release builds as well,
let's rename them to VERIFY to prevent confusion, as everyone is
used to assertions being compiled out in release.

We can introduce a new ASSERT macro that is specifically for debug
checks, but I'm doing this wholesale conversion first since we've
accumulated thousands of these already, and it's not immediately
obvious which ones are suitable for ASSERT.

Userland/Libraries/LibWeb/DOM/Window.cpp

@@ -95,7 +95,7 @@ i32 Window::set_timeout(JS::Function& callback, i32 interval)
 void Window::timer_did_fire(Badge<Timer>, Timer& timer)
 {
     // We should not be here if there's no JS wrapper for the Window object.
-    ASSERT(wrapper());
+    VERIFY(wrapper());
     auto& vm = wrapper()->vm();
 
     // NOTE: This protector pointer keeps the timer alive until the end of this function no matter what.