Add help command (#53)

Co-authored-by: nova <novanai.dev@gmail.com>

src/extensions/help.py

@@ -0,0 +1,45 @@
+import hikari
+import arc
+import itertools
+import collections
+
+plugin = arc.GatewayPlugin(name="Help Command Plugin")
+
+
+def gather_commands() -> dict[str | None, list[str]]:
+    plugin_commands: dict[str | None, list[str]] = collections.defaultdict(list)
+
+    for plugin_, commands in itertools.groupby(
+        plugin.client.walk_commands(hikari.CommandType.SLASH),
+        key=lambda cmd: cmd.plugin,
+    ):
+        for cmd in commands:
+            if not isinstance(cmd, (arc.SlashCommand, arc.SlashSubCommand)):
+                continue
+
+            plugin_commands[plugin_.name if plugin_ else None].append(
+                f"{cmd.make_mention()} - {cmd.description}"
+            )
+
+    return plugin_commands
+
+
+@plugin.include
+@arc.slash_command("help", "Displays a list of all commands.")
+async def help_command(ctx: arc.GatewayContext) -> None:
+    """Displays a simple list of all bot commands."""
+
+    plugin_commands = gather_commands()
+    embed = hikari.Embed(title="Bot Commands", color=0x00FF00)
+
+    for plugin_, commands in plugin_commands.items():
+        embed.add_field(
+            name=plugin_ or "No plugin", value="\n".join(commands), inline=False
+        )
+
+    await ctx.respond(embed=embed)
+
+
+@arc.loader
+def load(client: arc.GatewayClient) -> None:
+    client.add_plugin(plugin)