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/CMakeLists.txt

@@ -30,6 +30,7 @@ add_subdirectory(LibJS)
 add_subdirectory(LibKeyboard)
 add_subdirectory(LibLine)
 add_subdirectory(LibM)
+add_subdirectory(LibMain)
 add_subdirectory(LibMarkdown)
 add_subdirectory(LibPCIDB)
 add_subdirectory(LibPDF)