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.cpp

@@ -0,0 +1,29 @@
+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/Format.h>
+#include <AK/StringView.h>
+#include <AK/Vector.h>
+#include <LibMain/Main.h>
+
+int main(int argc, char** argv)
+{
+    Vector<StringView> arguments;
+    arguments.ensure_capacity(argc);
+    for (int i = 0; i < argc; ++i)
+        arguments.unchecked_append(argv[i]);
+
+    auto result = serenity_main({
+        .argc = argc,
+        .argv = argv,
+        .arguments = arguments.span(),
+    });
+    if (result.is_error()) {
+        warnln("Runtime error: {}", result.error());
+        return 1;
+    }
+    return result.value();
+}