command: add commandGroup function

This just creates an empty command with an auto-assigned noop callback.
This is useful sugar for creating a group of commands under a common
name because previously the user would have to define their own noop
callback and bind it. This just takes a description string
(and, optionally, a help flag override).

source/command.zig

@@ -72,6 +72,21 @@ fn BuilderGenerics(comptime UserContext: type) type {
     };
 }
 
+pub const GroupOptions = struct {
+    help_flag: ShortLongPair = .{ .short_tag = "-h", .long_tag = "--help" },
+    description: []const u8,
+};
+
+pub fn commandGroup(allocator: std.mem.Allocator, comptime options: GroupOptions) !ParserInterface {
+    const cmd = comptime CommandBuilder(void){
+        .help_flag = options.help_flag,
+        .description = options.description,
+        .subcommand_required = true,
+    };
+
+    return try cmd.createInterface(allocator, cmd.noopCallback());
+}
+
 fn InterfaceCreator(comptime Command: type) type {
     return if (Command.ICC.InputType()) |Type|
         struct {
@@ -329,6 +344,12 @@ pub fn CommandBuilder(comptime UserContext: type) type {
             return self.param_spec.realTuple();
         }
 
+        pub fn noopCallback(comptime self: @This()) self.CallbackSignature() {
+            return struct {
+                fn callback(_: UserContextType, _: self.Output()) !void {}
+            }.callback;
+        }
+
         pub fn CallbackSignature(comptime self: @This()) type {
             return *const fn (UserContextType, self.Output()) anyerror!void;
         }

source/noclip.zig

@@ -7,4 +7,5 @@ pub const parameters = @import("./parameters.zig");
 pub const parser = @import("./parser.zig");
 
 pub const CommandBuilder = command.CommandBuilder;
+pub const commandGroup = command.commandGroup;
 pub const ParserInterface = parser.ParserInterface;