Add a minimal chat example (#248)

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## Description

Take 3 of #247.

I've added a minimal chat example - ignoring chat signing, etc. It's in
the file `crates/valence/examples/chat.rs`.

## Test Plan

Manual, unfortunately :(

Steps:
1. Open two minecraft clients, with two separate accounts (or usernames
if using offline mode)
2. Connect to the server in both clients
3. Type a chat message, checking it appears to both clients, with the
correct username.
4. Type a command, checking it appears only to the sender.

---------

Co-authored-by: Carson McManus <dyc3@users.noreply.github.com>

crates/valence/examples/chat.rs

@@ -0,0 +1,90 @@
+use bevy_app::App;
+use tracing::warn;
+use valence::client::despawn_disconnected_clients;
+use valence::client::event::{default_event_handler, ChatCommand, ChatMessage};
+use valence::prelude::*;
+
+const SPAWN_Y: i32 = 64;
+
+pub fn main() {
+    tracing_subscriber::fmt().init();
+
+    App::new()
+        .add_plugin(ServerPlugin::new(()).with_connection_mode(ConnectionMode::Offline))
+        .add_startup_system(setup)
+        .add_system_to_stage(EventLoop, default_event_handler)
+        .add_system_to_stage(EventLoop, handle_message_events)
+        .add_system_to_stage(EventLoop, handle_command_events)
+        .add_system(init_clients)
+        .add_system(despawn_disconnected_clients)
+        .add_system_set(PlayerList::default_system_set())
+        .run();
+}
+
+fn setup(world: &mut World) {
+    let mut instance = world
+        .resource::<Server>()
+        .new_instance(DimensionId::default());
+
+    for z in -5..5 {
+        for x in -5..5 {
+            instance.insert_chunk([x, z], Chunk::default());
+        }
+    }
+
+    for z in -25..25 {
+        for x in -25..25 {
+            instance.set_block([x, SPAWN_Y, z], BlockState::BEDROCK);
+        }
+    }
+
+    world.spawn(instance);
+}
+
+fn init_clients(
+    mut clients: Query<&mut Client, Added<Client>>,
+    instances: Query<Entity, With<Instance>>,
+) {
+    for mut client in &mut clients {
+        client.set_position([0.0, SPAWN_Y as f64 + 1.0, 0.0]);
+        client.set_instance(instances.single());
+        client.set_game_mode(GameMode::Adventure);
+        client.send_message("Welcome to Valence! Talk about something.".italic());
+    }
+}
+
+fn handle_message_events(mut clients: Query<&mut Client>, mut messages: EventReader<ChatMessage>) {
+    for message in messages.iter() {
+        let Ok(client) = clients.get_component::<Client>(message.client) else {
+            warn!("Unable to find client for message: {:?}", message);
+            continue;
+        };
+
+        let message = message.message.to_string();
+
+        let formatted = format!("<{}>: ", client.username())
+            .bold()
+            .color(Color::YELLOW)
+            + message.into_text().not_bold().color(Color::WHITE);
+
+        clients.par_for_each_mut(16, |mut client| {
+            client.send_message(formatted.clone());
+        })
+    }
+}
+
+fn handle_command_events(mut clients: Query<&mut Client>, mut commands: EventReader<ChatCommand>) {
+    for command in commands.iter() {
+        let Ok(mut client) = clients.get_component_mut::<Client>(command.client) else {
+            warn!("Unable to find client for message: {:?}", command);
+            continue;
+        };
+
+        let message = command.command.to_string();
+
+        let formatted =
+            "You sent the command ".into_text() + ("/".into_text() + (message).into_text()).bold();
+
+        client.send_message(formatted);
+    }
+}