Support abstract [LuaObject] classes

[ashtonmeuser]

The guard against abstract classes is, in my opinion, a little conservative. Users who desire an abstract parent class may simply drop abstract to appease Lua-CSharp, which is a worse compromise.

If I've overlooked a massive trap, please let me know.

[ashtonmeuser]

Perhaps this isn't necessary. The abstract parent class can simply omit [LuaObject] and add [LuaMember] all the same. A concrete child class with [LuaObject] will pick these up. For example:

public abstract partial class ParentClass
{
    [LuaMember("value")]
    public string Value { get; set; } = "";
}

[LuaObject]
public partial class ChildClass : ParentClass
{
    [LuaMember("number")]
    public int Number { get; set; }
}

ChildClass still has access to value.