LibMain: Add a new library for more ergonomic userspace entry functions

By linking with LibMain, your program no longer needs to provide main().
Instead, execution begins in this function:

    ErrorOr<int> serenity_main(Main::Arguments);

This allows programs that link with LibMain to use TRY() already in
their entry function, without having to do manual ErrorOr unwrapping.

This is very experimental, but it seems like a nice idea so let's try it
out. :^)

Userland/Libraries/LibMain/Main.h

@@ -0,0 +1,21 @@
+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Error.h>
+
+namespace Main {
+
+struct Arguments {
+    int argc {};
+    char** argv {};
+    Span<StringView> arguments;
+};
+
+}
+
+ErrorOr<int> serenity_main(Main::Arguments);