feat: add channel playlists

- add tests for channel videos
- small model refactor (rename Channel to ChannelTag)

src/model/param.rs

@@ -0,0 +1,11 @@
+#[derive(Default, Debug, Copy, Clone, PartialEq, Eq)]
+#[non_exhaustive]
+pub enum ChannelOrder {
+    /// Output the latest videos first
+    #[default]
+    Latest,
+    /// Output the oldest videos first
+    Oldest,
+    /// Output the most viewed videos first
+    Popular,
+}