Breaking: runtime-deprecation on '~/.eslintrc' (refs eslint/rfcs#32)

[mysticatea]

Don't merge until we finished releasing the last one of 6.x.

What is the purpose of this pull request? (put an "X" next to item)

[X] Add something to the core

What changes did you make? (Give an overview)

This PR adds runtime-deprecation about personal config files (~/.eslintrc.*). This is a part of RFC32.

ESLint emits deprecation warnings when:

• when ESLint loaded a personal config file.
• when ESLint ignored a personal config file because of project config files that don't have root:true.

The warning messages are like:

(node:20312) [ESLINT_PERSONAL_CONFOG_LOAD] DeprecationWarning: The '~/.eslintrc.*' config file has been deprecated. Please use config files for each project or '--config' option. (found in "eslint\fixtures\config-hierarchy\personal-config\home-folder\.eslintrc.json")

(node:20312) [ESLINT_PERSONAL_CONFOG_SUPPRESS] DeprecationWarning: The '~/.eslintrc.*' config file has been deprecated. Please remove it or add 'root:true' into the config file of your projects in order to avoid loading '~/.eslintrc.*' accidentally. (found in "eslint\fixtures\config-hierarchy\personal-config\home-folder\.eslintrc.json")

Is there anything you'd like reviewers to focus on?

The warning messages are understandable?

[kaicataldo]

Would it be useful to add some tests around this change?

[mysticatea]

Thank you for your review. I have updated this PR.

[mysticatea]

I added some tests. It clarifies that ESLint raises new deprecation warnings only when the cases that ESLint change the behavior after the personal config file feature were removed.

[kaicataldo]

LGTM, thank you!