added IF NOT EXISTS to create and CRUD for each table

backend/database/sql/queries/artist_credit_artists.sql

@@ -0,0 +1,17 @@
+-- name: CreateArtistCreditArtist :one
+INSERT INTO artist_credit_artist (artist_id, credit_id) VALUES (?, ?)
+RETURNING *;
+
+-- name: GetArtistCreditArtist :one
+SELECT * FROM artist_credit_artist 
+WHERE id = ? LIMIT 1;
+
+-- name: UpdateArtistCreditArtist :exec
+UPDATE artist_credit_artist 
+SET artist_id = ?, credit_id = ?
+WHERE id =?;
+
+-- name: DeleteArtistCreditArtist :exec
+DELETE FROM artist_credit_artist 
+WHERE id =?;
+