support sorting on tags

I don't personally understand it, but some users seem to want it.

rslib/src/config.rs

@@ -42,6 +42,7 @@ pub enum SortKind {
     NoteField,
     #[serde(rename = "note")]
     NoteType,
+    NoteTags,
     CardMod,
     CardReps,
     CardDue,

rslib/src/search/cards.rs

@@ -51,6 +51,7 @@ fn write_order(sql: &mut String, kind: &SortKind, reverse: bool) -> Result<()> {
         }
         SortKind::CardLapses => "c.lapses",
         SortKind::CardInterval => "c.ivl",
+        SortKind::NoteTags => "n.tags",
         SortKind::CardDeck => "(select v from sort_order where k = c.did)",
         SortKind::NoteType => "(select v from sort_order where k = n.mid)",
         SortKind::CardTemplate => "(select v from sort_order where k1 = n.mid and k2 = c.ord)",