AK+Kernel+Userland: Don't allow Vector::append() in the kernel

This makes it not as easy to forgot to handle OOMs in the kernel.

This commit replaces most usages of this function with
`try_append(...).release_value_but_fixme_should_propagate_errors()`.
But in some cases, using the `TRY` macro or `unchecked_append()` is
already possible.

In places where allocations should not fail or an OOM would be fatal
anyways, `MUST(try_append(...))` should be used explicitly.

Kernel/FileSystem/VirtualFileSystem.cpp

@@ -273,7 +273,7 @@ void VirtualFileSystem::sync_filesystems()
     Vector<NonnullRefPtr<FileSystem>, 32> file_systems;
     s_details->file_systems_list.with([&](auto const& list) {
         for (auto& fs : list)
-            file_systems.append(fs);
+            file_systems.try_append(fs).release_value_but_fixme_should_propagate_errors();
     });
 
     for (auto& fs : file_systems) {