add environmental variables and set up secret detection

[zxt-tzx]

First commit closes #1261.

• Issue: currently npm run dev is not working because env-cmd is expecting an env file in the root directory
• Solution: It adds an .env.example file to the backend, updates the README to tell the user to duplicate a .env file. After doing so, npm run dev will work properly.

Second commit is based on Postman's PR opengovsg/postmangovsg#1554, which was made after we our previous secret detection linter failed to pick up secret. We experimented with a few options and detect-secrets work the best.

• Issue: currently, ts-template does not have secret detection
• Solution: add detect-secrets

Not sure if I've set up the ci.yml and husky hooks properly — would be good to have someone look through them!

[zxt-tzx]

the GitHub Actions ci seems to be failing here:

hmm

[LoneRifle]

Can you rework this so that it's inline with current repo state?

• We don't need .env.example anymore, given that we now have .env.development, so you can drop the first commit.
• Is there a node equivalent to detect-secrets?