Add eslint

[timja]

Some suggestion for pure javascript

Hook up eslint and prettier and unicorn. Unicorn has sane defaults for modern javascript.
See my PR here updatecli/updatecli-action#100

Add jsconfig.json should help with improve type detection for vscode.
Add jsconfig.json in your test folder that helps with jest type annotations.

index.js should really only be routing probot.

Everything else should be imported. FYI probot blocks you from switching to ESM. So stick with commonjs due to: probot/pino#188
Because of it you need to turn off some features of unicorn, see release-drafter eslintrc

Originally posted by @jetersen in #7 (comment)