Added public SteamInterface interface

[UnKabaraQuiDev]

Issue:
com.codedisaster.steamworks.SteamInterface was package-private so this wasn't previously possible:

Map<Class<? extends SteamInterface>, SteamInterface> STEAM_INTERFACES

I only need to reference the dispose() method from SteamInterface:

public static void dispose() {
	STEAM_INTERFACES.values().forEach(e -> e.dispose());
	SteamAPI.shutdown();
}

I solved it by creating my class in the package com.codedisaster.steamworks but this is a bit clunky and I don't like having multiple root packages in a same project.

Solutions:
I renamed SteamInterface to SteamInterfaceImpl and kept it package-private. Created a new interface SteamInterface and implemented it in SteamInterfaceImpl, it only declares a void dispose().

Also added some javadoc