command: declare command interface

Signed-off-by: Ophestra <cat@gensokyo.uk>

command/command.go

@@ -0,0 +1,34 @@
+// Package command implements generic nested command parsing.
+package command
+
+import "flag"
+
+type (
+	// HandlerFunc is called when matching a directly handled subcommand tree.
+	HandlerFunc = func(args []string) error
+
+	// LogFunc is the function signature of a printf function.
+	LogFunc = func(format string, a ...any)
+
+	// FlagDefiner is a deferred flag definer value, usually encapsulating the default value.
+	FlagDefiner interface {
+		// Define defines the flag in set.
+		Define(set *flag.FlagSet, p any, name, usage string)
+	}
+
+	Command interface {
+		Parse(arguments []string) error
+		baseNode[Command]
+	}
+	Node baseNode[Node]
+
+	baseNode[T any] interface {
+		// Command appends a subcommand with direct command handling.
+		Command(name, usage string, f HandlerFunc) T
+		// Flag defines a generic flag type in Node's flag set.
+		Flag(p any, name string, value FlagDefiner, usage string) T
+
+		// New returns a new subcommand tree.
+		New(name, usage string) (sub Node)
+	}
+)