Drop support for ESLint<v6

[ota-meshi]

Drop support for ESLint < v6. This will allow us to use @eslint-community/eslint-utils.
https://github.com/eslint-community/eslint-utils

However, it is a breaking change and must be made in a major version. Before that, I want to resolve and merge #24 and #26. Because we need to change the shareable configuration.
We also need a fork of regexpp for that.