AK: Use direct-list-initialization for Vector::empend() (#4564)

clang trunk with -std=c++20 doesn't seem to properly look for an
aggregate initializer here when the type being constructed is a simple
aggregate (e.g. `struct Thing { int a; int b; };`). This template fails
to compile in a usage added 12/16/2020 in `AK/Trie.h`.

Both forms of initialization are supposed to call the
aggregate-initializers but direct-list-initialization delegating to
aggregate initializers is a new addition in c++20 that might not be
implemented yet.

AK/Vector.h

@@ -398,7 +398,7 @@ public:
     void empend(Args&&... args)
     {
         grow_capacity(m_size + 1);
-        new (slot(m_size)) T(forward<Args>(args)...);
+        new (slot(m_size)) T { forward<Args>(args)... };
         ++m_size;
     }