feat: add support for ESLint Flat Config

[k1tikurisu]

an example:

const config = require("@cybozu/eslint-config/flat/presets/react-typescript-prettier")

module.exports = [
  ...config,
  {
    rules: {
      // default
      // 'indent': ['warn', 2, { "SwitchCase": 1 }],
      indent: ["warn", 4, { SwitchCase: 0 }]
    }
  },
]