feat!: React 17 support

[nissy-dev]

Fixed #450

I added "plugin:react/jsx-runtime" for React 17 based on the following statement.

If you are using the new JSX transform from React 17, extend react/jsx-runtime in your eslint config (add "plugin:react/jsx-runtime" to "extends") to disable the relevant rules.

https://github.com/yannickcr/eslint-plugin-react#configuration

[koba04]

👍 LGTM!!!

[koba04]

Could you fix failing tests?

[nissy-dev]

@koba04
I think there are two ways of passing the tests.

1. just removing import React from "react" in tests
2. enable "react/jsx-uses-react" rules

Which is better? I seem that 1. way leads to breaking backward compatibility.
Some projects should remove all unused React import statements.

[sakit0]

LGTM🎉

[koba04]

@nissy-dev I prefer the option 1.
This is definitely a breaking change, but React v17 was released over a year, so I think it makes sense to drop React v16 support. We can use the codemod script the React team provides for the migration.

https://reactjs.org/blog/2020/09/22/introducing-the-new-jsx-transform.html#removing-unused-react-imports

But we should provide an announcement for the breaking change that we can still use the classic JSX syntax (React.createElement) by enabling the react/jsx-uses-react rule.
So let's add a note for the classic JSX syntax in README.

---

⚠️ Class JSX Syntax

@cybozu/eslint-config is intented to be used with the New JSX Transform. If you want to use the Classic JSX Transform (React.createElement), please enable the react/jsx-uses-react rule on your own.

rules: {
  "react/jsx-uses-react": "error"
}

[koba04]

@nissy-dev
Thank you! I have an additional request 🙏
I want to add the ## React Support section and move this to the upper of the ## For kintone customize developers section.

Suggested change

	## ⚠️ Class JSX Syntax
	## React Support
	### ⚠️ Classic JSX Syntax

Could you apply the change and move the section to the upper of the ## For kintone customize developers section?

[nissy-dev]

@koba04
I fixed!

[koba04]

👏 👏 👏 Great works!!!