Enable eslint for CLI package

[bajtos]

The package packages/cli is the only package written in JavaScript and because it's an exception in our TypeScript-based project, it's not covered by any linter. As a result, this part of our codebase may contain silly bugs that could have been caught by a linter.

I am proposing to start running eslint for packages/cli.

Alternatively, we can rewrite @loopback/cli into TypeScript, but that would be a larger task IMO.

Acceptance criteria

• npm run lint executes eslint for packages/cli
• npm run lint:fix executes eslint --fix
• eslint is configured using eslint-config-loopback with custom configuration to disable eslint rules clashing with our Prettier formatting.
• all linting errors are fixed - this should be a standalone commit in the pull request.

[raymondfeng]

1. +1 to use eslint as the short-term improvement.

2. Converting the package into TypeScript might not be too complicated as we have used ES6 classes for most of the code.

[bajtos]

eslint is configured using eslint-config-loopback with custom configuration to disable eslint rules clashing with our Prettier formatting.

We can use eslint-plugin-prettier or eslint-config-prettier to help with that.

[bajtos]

Let's wait until #2608 is done.

[dhmlau]

Closing it as done.