Enable code linter in juggler monorepo

[bajtos]

_This story requires the new monorepo to be created first, see #1725, and also Prettier to be already configured, see #1740.

Start linting code using eslint. If we decide to migrate to TypeScript before this story is done, then use tslint instead of eslint, or whatever approach is used by loopback-next at that time.

Caveat: eslint-config-loopback contains rules that are duplicating Prettier formatting. Ideally, we should modify eslint-config-loopback to be fully compatible with prettier. Alternatively, we can disable formatting-related rules in .eslintrc of our new monorepo.

Acceptance criteria

The changes should be split to (at least) the following two commits:

Setup eslint infrastructure

• Add .eslintrc, see the version imported from loopback-datasource-juggler for inspiration.
• Remove .eslintrc files in individual packages (juggler, filters, etc.)
• Add eslint to both npm run lint and npm run lint:fix. Use .gitignore as the file to configure list of source code files to ignore (see how linting was setup in original juggler). Important: run eslint first, prettier second.
• Remove linting scripts from package.json files of individual packages/connectors.

Reformat the code

• Execute npm run lint:fix to reformat the code. Since we were already using eslint in the legacy codebase, this should hopefully introduce very few changes.

[stale]

This issue has been marked stale because it has not seen activity within six months. If you believe this to be in error, please contact one of the code owners, listed in the CODEOWNERS file at the top-level of this repository. This issue will be closed within 30 days of being stale.

[stale]

This issue has been closed due to continued inactivity. Thank you for your understanding. If you believe this to be in error, please contact one of the code owners, listed in the CODEOWNERS file at the top-level of this repository.