improvement: support {:unsafe_fragment, ".."} as a conflict target

this is supported by other sql data layers, for example. This is necessary for AshSqlite3's upsert behavior.
Postgres adapter implementation: https://github.com/elixir-ecto/ecto_sql/blob/v3.10.2/lib/ecto/adapters/postgres/connection.ex#L212C4-L213C26

lib/ecto/adapters/sqlite3/connection.ex

@@ -748,6 +748,9 @@ defmodule Ecto.Adapters.SQLite3.Connection do
 
   defp conflict_target([]), do: ""
 
+  defp conflict_target({:unsafe_fragment, fragment}),
+    do: [fragment, ?\s]
+
   defp conflict_target(targets) do
     [?(, intersperse_map(targets, ?,, &quote_name/1), ?), ?\s]
   end