Setup a basic SystemSet enum for the game

[janhohenheim]

Migrated from bevyengine/bevy_github_ci_template#63

Same rationale as #4

[benfrankel]

This is big bikeshed territory. I can share my own game logic system set enum:

/// (Update) Game logic system ordering
#[derive(SystemSet, Clone, Eq, PartialEq, Hash, Debug)]
pub enum UpdateSet {
    /// Handle actions pressed this frame
    HandleActions,
    /// Apply deferred effects from HandleActions
    HandleActionsFlush,
    /// Initialize start-of-frame values and tick timers
    Start,
    /// Step game logic
    Update,
    /// Run the trigger-effect system
    React,
    /// Record player and AI intents
    RecordIntents,
    /// Apply player and AI intents
    ApplyIntents,
    /// Handle events emitted this frame
    HandleEvents,
    /// Queue despawn commands from DespawnSet
    QueueDespawn,
    /// Apply spawn / despawn and other commands
    ApplyDeferred,
    /// Update UI
    UpdateUi,
    /// Synchronize end-of-frame values
    End,
}

This could definitely be stripped down. I'm curious if there are commonalities between the system set enums in different template repos.

[MiniaczQ]

It's hard to setup any ordering without providing game logic, closely bound to #11