chore: add .env.example and ignore generated editor config

Commit the .env.example template and exempt it from the .env ignore rule.
Ignore the .sqls.yml file that sqlc generate emits for editor tooling.

.env.example

@@ -0,0 +1,4 @@
+DB_URL=postgres://postgres:postgres@localhost:5432/chirpy?sslmode=disable
+PLATFORM=dev
+JWT_SECRET=replace-with-a-long-random-secret
+POLKA_KEY=replace-with-your-polka-api-key

.gitignore

@@ -1,2 +1,4 @@
 bin/
 *.env*
+!.env.example
+.sqls.yml