slag: support cobra's pflag interface (#24)

Signed-off-by: Jason Hall <[email protected]>

slag/flag.go

@@ -13,7 +13,18 @@
 //
 //	$ ./myprogram -log-level=debug
 //
-// The slag.Level type is a wrapper around slog.Level that implements the flag.Value interface.
+// The slag.Level type is a wrapper around slog.Level that implements the flag.Value interface,
+// as well as Cobra's pflag.Value interface.
+//
+//	func main() {
+//		var level slag.Level
+//		cmd := &cobra.Command{
+//			Use: "myprogram",
+//			...
+//		}
+//		cmd.PersistentFlags().Var(&level, "log-level", "log level")
+//		cmd.Execute()
+//	}
 package slag
 
 import "log/slog"
@@ -30,3 +41,6 @@ func (l *Level) Set(s string) error {
 }
 func (l *Level) String() string    { return slog.Level(*l).String() }
 func (l *Level) Level() slog.Level { return slog.Level(*l) }
+
+// Implements https://pkg.go.dev/github.com/spf13/pflag#Value
+func (l *Level) Type() string { return "string" }