refactor: Add index on country.name

See: https://github.com/JetBrains/skills/blob/main/postgres-best-practices/references/query-missing-indexes.md

Refs: #92

Author: ghusta

scripts/world.sql

@@ -121,6 +121,7 @@ ALTER TABLE city
 
 -- Added in 2.15
 CREATE INDEX city_name_idx ON city (name);
+CREATE INDEX country_name_idx ON country (name);
 
 COMMIT;