refactor: move session package to internal

internal/session/slug/slug.go

@@ -0,0 +1,24 @@
+package slug
+
+type Slug interface {
+	String() string
+	Set(slug string)
+}
+
+type slug struct {
+	slug string
+}
+
+func New() Slug {
+	return &slug{
+		slug: "",
+	}
+}
+
+func (s *slug) String() string {
+	return s.slug
+}
+
+func (s *slug) Set(slug string) {
+	s.slug = slug
+}