LibCore: Introduce a new directory iteration API

`Core::Directory::for_each_entry()` takes a callback which is passed the
DirectoryEntry and the parent Directory. It returns any error from
creating the iterator, iterating the entries, or returned from the
callback.

As a simple example, this:

```c++
Core::DirIterator piece_set_iterator { "/res/icons/chess/sets/",
        Core::DirIterator::SkipParentAndBaseDir };
while (piece_set_iterator.has_next())
    m_piece_sets.append(piece_set_iterator.next_path());
```

becomes this:

```c++
TRY(Core::Directory::for_each_entry("/res/icons/chess/sets/"sv,
        Core::DirIterator::SkipParentAndBaseDir,
        [&](auto const& entry, auto&) -> ErrorOr<IterationDecision> {
    TRY(m_piece_sets.try_append(entry.name));
    return IterationDecision::Continue;
}));
```

Userland/Libraries/LibCore/Directory.cpp

@@ -93,4 +93,32 @@ ErrorOr<struct stat> Directory::stat() const
     return System::fstat(m_directory_fd);
 }
 
+ErrorOr<void> Directory::for_each_entry(DirIterator::Flags flags, Core::Directory::ForEachEntryCallback callback)
+{
+    DirIterator iterator { path().string(), flags };
+    if (iterator.has_error())
+        return iterator.error();
+
+    while (iterator.has_next()) {
+        if (iterator.has_error())
+            return iterator.error();
+
+        auto entry = iterator.next();
+        if (!entry.has_value())
+            break;
+
+        auto decision = TRY(callback(entry.value(), *this));
+        if (decision == IterationDecision::Break)
+            break;
+    }
+
+    return {};
+}
+
+ErrorOr<void> Directory::for_each_entry(AK::StringView path, DirIterator::Flags flags, Core::Directory::ForEachEntryCallback callback)
+{
+    auto directory = TRY(Directory::create(path, CreateDirectories::No));
+    return directory.for_each_entry(flags, move(callback));
+}
+
 }