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/CMakeLists.txt

@@ -0,0 +1,6 @@
+set(SOURCES
+    Main.cpp
+)
+
+serenity_lib(LibMain main)
+target_link_libraries(LibMain LibC)