Upgrade to ESLint 6

[Paolo Caleffi]

This is more a whole-project stuff, but here I'm gonna list dependencies which need to update to ESLint6 to guarantee compatiblity and related links.

Note that ESLint currently works out-of-the-box even with older dependencies, but it fires off a warning.

• @typescript-eslint/eslint-plugin and @typescript-eslint/parser will support ESLint 6 in a couple of weeks #23
• @vue/eslint-plugin-vue will support ESLint 6 in next release vuejs/eslint-plugin-vue#920
• eslint-config-prettier already supports ESLint 6
• eslint-loader seems to already support ESLint 6, but I cannot be sure because Github is having some severe server failure right now

[Paolo Caleffi]

ESLint 6 is now supported