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;
}));
```

1 files changed, 9 insertions, 2 deletions

diff --git a/Userland/Libraries/LibCore/Directory.h b/Userland/Libraries/LibCore/Directory.h
index ccc97e0714..ec04cc714a 100644
--- a/Userland/Libraries/LibCore/Directory.h
+++ b/Userland/Libraries/LibCore/Directory.h
@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2022, kleines Filmröllchen <filmroellchen@serenityos.org>
+ * Copyright (c) 2023, Sam Atkins <atkinssj@serenityos.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -8,17 +9,19 @@
 
 #include <AK/Error.h>
 #include <AK/Format.h>
+#include <AK/Function.h>
+#include <AK/IterationDecision.h>
 #include <AK/LexicalPath.h>
 #include <AK/Noncopyable.h>
 #include <AK/Optional.h>
+#include <LibCore/DirIterator.h>
+#include <LibCore/DirectoryEntry.h>
 #include <LibCore/File.h>
 #include <dirent.h>
 #include <sys/stat.h>
 
 namespace Core {
 
-class DirIterator;
-
 // Deal with real system directories. Any Directory instance always refers to a valid existing directory.
 class Directory {
     AK_MAKE_NONCOPYABLE(Directory);
@@ -43,6 +46,10 @@ public:
 
     LexicalPath const& path() const { return m_path; }
 
+    using ForEachEntryCallback = Function<ErrorOr<IterationDecision>(DirectoryEntry const&, Directory const& parent)>;
+    static ErrorOr<void> for_each_entry(StringView path, DirIterator::Flags, ForEachEntryCallback);
+    ErrorOr<void> for_each_entry(DirIterator::Flags, ForEachEntryCallback);
+
     ErrorOr<void> chown(uid_t, gid_t);
 
     static ErrorOr<bool> is_valid_directory(int fd);