Split message content into its own module

This is laying some groundwork for the rich text chatbar.

src/messagecontent/config-neochat.h.in

@@ -0,0 +1,8 @@
+/*
+   SPDX-FileCopyrightText: 2024 Tobias Fella <tobias.fella@kde.org>
+   SPDX-License-Identifier: LGPL-2.0-or-later
+*/
+
+#pragma once
+
+#define CMAKE_INSTALL_FULL_LIBEXECDIR_KF6 "${KDE_INSTALL_FULL_LIBEXECDIR_KF}"