LibDiff: Add Diff::apply_patch

Given a set of lines from the file we are patching, and a patch itself,
this function will try and locate where in the file to apply that patch,
and write the result of patching that file (if successful) to the output
stream.

Userland/Libraries/LibDiff/Applier.h

@@ -0,0 +1,16 @@
+/*
+ * Copyright (c) 2023, Shannon Booth <shannon.ml.booth@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Error.h>
+#include <LibDiff/Forward.h>
+
+namespace Diff {
+
+ErrorOr<void> apply_patch(Stream& out, Vector<StringView> const& lines, Patch const& patch);
+
+}