• removes corelib dependence on AsyncEvent`2 for dispatching events and instead replaces it with a type-agnostic dispatcher assuming immutable events (removing most of the cruft from AsyncEvent`2 is also nice for perf)
• enables event handlers expressed as types by implementing IEventHandler<TEventArgs>. these can be registered using EventHandlingBuilder.AddEventHandlers<T>(ServiceLifetime); a type can contain an indefinite amount of handlers and the type will have a respected service lifetime according to the parameter.
• fixes a memory leak in Interactivity
  • interactivity generally is rather janky and remains rather janky here
• changes every extension setup to be along the lines of IServiceCollection.AddExtension(Action<Extension> setup, ExtensionConfiguration);, with matching methods on DiscordClientBuilder, following an UseExtension scheme according to past convention (open to changing this).
  • the setup parameter fills the need for calling additional configuration on the extension, so for example the following code would be refactored as follows
  • in the future, extensions fully supported throughout the v5 lifecycle should add first-class support without needing a setup parameter

// old
DiscordClientBuilder builder = // we get a client here;
DiscordClient client = builder.Build();

CommandsExtension extension = client.UseCommands(myConfig);
extension.AddCommands([typeof(Command)]);
extension.AddConverters(...);

await client.ConnectAsync();

// new
DiscordClientBuilder builder = // we get a client here;
builder.UseCommands(extension => 
    {
        extension.AddCommands([typeof(MyCommand)]);
        extension.AddConverters(...);
    },
    myConfig
);

await builder.ConnectAsync();

closes #1820 as complete