added IF NOT EXISTS to create and CRUD for each table

backend/database/sql/queries/artist_credit.sql

@@ -0,0 +1,17 @@
+-- name: CreateArtistCredit :one
+INSERT INTO artist_credit (text) VALUES (?)
+RETURNING *;
+
+-- name: GetArtistCredit :one
+SELECT * FROM artist_credit 
+WHERE id = ? LIMIT 1;
+
+-- name: UpdateArtistCredit :exec
+UPDATE artist_credit 
+SET text = ?
+WHERE id =?;
+
+-- name: DeleteArtistCredit :exec
+DELETE FROM artist_credit 
+WHERE id =?;
+