Support custom IsZero() methods with omitzero tag (#1020)

The omitzero tag now respects custom IsZero() methods on types,
similar to how encoding/json handles this. Previously, only
reflect.Value.IsZero() was used, which ignores user-defined
implementations.

Fixes #1003

Co-authored-by: Claude <noreply@anthropic.com>

types.go

@@ -6,10 +6,17 @@ import (
 	"time"
 )
 
+// isZeroer is used to check if a type has a custom IsZero method.
+// This allows custom types to define their own zero-value semantics.
+type isZeroer interface {
+	IsZero() bool
+}
+
 var (
 	timeType               = reflect.TypeOf((*time.Time)(nil)).Elem()
 	textMarshalerType      = reflect.TypeOf((*encoding.TextMarshaler)(nil)).Elem()
 	textUnmarshalerType    = reflect.TypeOf((*encoding.TextUnmarshaler)(nil)).Elem()
+	isZeroerType           = reflect.TypeOf((*isZeroer)(nil)).Elem()
 	mapStringInterfaceType = reflect.TypeOf(map[string]interface{}(nil))
 	sliceInterfaceType     = reflect.TypeOf([]interface{}(nil))
 	stringType             = reflect.TypeOf("")