feat: add support for ESLint 7, drop support ESLint 5

[Kocal]

• bump eslint from ^5.15.2 || ^6.0.0 to ^6.0.0 || ^7.0.0: https://eslint.org/blog/2020/05/eslint-v7.0.0-released
• bump eslint-loader from ^3.0.0 to ^4.0.0, which drop support for Node <10.13.0 and set minimum ESLint support to version 6: https://github.com/webpack-contrib/eslint-loader/blob/master/CHANGELOG.md#400-2020-04-03
• had to fix some linting issues

EDIT: Ok so ESLint 7 is not compatible with Node 11.
This version is in end of life (End-of-Life releasese, should we drop support for Node 11 too? And maybe add support for Node 13 and Node 14?
WDYT?

EDIT 2: PR opened at #775

EDIT 3: PR rebased since #775 has been merged

[Kocal]

PR rebased since #775 has been merged.
The failing test is unrelated I guess.

[weaverryan]

Thanks @Kocal!