Improved ESLint setup

[DenysVuika]

Please check if the PR fulfills these requirements

[x] The commit message follows our [guidelines](https://github.com/Alfresco/alfresco-js-api/wiki/Commit-format)
[ ] Tests for the changes have been added (for bug fixes / features)
[ ] Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (check one with "x")

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[x] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] Documentation
[ ] Other... Please describe:

What is the current behavior? (You can also link to an open issue here)

What is the new behavior?

• switch ESLint config to .json format
• disable certain warnings for the auto-generated content
• enable various rules for the "tests" folder

Does this PR introduce a breaking change? (check one with "x")

[ ] Yes
[x] No

If this PR contains a breaking change, please describe the impact and migration path for existing applications: ...

Other information: