Improve the UX of using an npm installed formatter

This is currently what you have to write to use a custom npm installed formatter with ESLint:

$ eslint --format=node_modules/eslint-formatter-unicorn file.js

That’s not very user-friendly.

I propose the following:

1. Make getFormatter() try to require the formatter from cwd first. That way you could just do:

$ eslint --format=eslint-formatter-unicorn file.js

1. Make it possible to omit the eslint-formatter prefix. This matches the convention for shareable configs. This combined with 1. would mean you could just do:

$ eslint --format=unicorn file.js

Much nicer! ✨

I would also document the recommendation of using the eslintformatter keyword in package.json, like you do with shareable configs. This would make it easier for users to find third-party formatters.

[ilyavolodin]

I was wondering if we should instead add formatters to be distributed through plugins? Formatters are the only thing we currently have that we don’t have a recommended way of shearing.

[not-an-aardvark]

This was added in https://github.com/eslint/eslint/pull/9464.