LibCore: Move FileWatcher implementations into separate files

Rather than one big file with (eventually) all implementations for each
OS, let's keep OS-specific implementations in their own files.

Userland/Libraries/LibCore/FileWatcherUnimplemented.cpp

@@ -0,0 +1,38 @@
+/*
+ * Copyright (c) 2020, Itamar S. <itamar8910@gmail.com>
+ * Copyright (c) 2021, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include "FileWatcher.h"
+#include <Kernel/API/InodeWatcherFlags.h>
+#include <LibCore/Notifier.h>
+#include <errno.h>
+
+namespace Core {
+
+ErrorOr<NonnullRefPtr<FileWatcher>> FileWatcher::create(InodeWatcherFlags)
+{
+    return Error::from_errno(ENOTSUP);
+}
+
+FileWatcher::FileWatcher(int watcher_fd, NonnullRefPtr<Notifier> notifier)
+    : FileWatcherBase(watcher_fd)
+    , m_notifier(move(notifier))
+{
+}
+
+FileWatcher::~FileWatcher() = default;
+
+ErrorOr<bool> FileWatcherBase::add_watch(DeprecatedString, FileWatcherEvent::Type)
+{
+    return Error::from_errno(ENOTSUP);
+}
+
+ErrorOr<bool> FileWatcherBase::remove_watch(DeprecatedString)
+{
+    return Error::from_errno(ENOTSUP);
+}
+
+}