Support for new eslint flat config

[IlyaSemenov]

The eslint flat config is the new recommended approach.

I believe this plugin should be updated to either support it alongside the deprecated eslintrc config, or switch to the new format fully.

---

For the reference, in the meanwhile, one can "manually" craft the config with something like:

import defineConfig from "@antfu/eslint-config"
import vuePug from "eslint-plugin-vue-pug"

export default defineConfig(
  {
    // Use @antfu/eslint-config for the sake of not configuring vue manually.
  },
  {
    // Combined from eslint-plugin-vue-pug/lib/configs/*.js
    name: "vue-pug",
    files: ["**/*.vue"],
    languageOptions: {
      parserOptions: {
        templateTokenizer: { pug: "vue-eslint-parser-template-tokenizer-pug" },
      },
    },
    plugins: {
      "vue-pug": vuePug,
    },
    rules: {
      // base
      "vue/component-name-in-template-casing": "off",
      "vue/html-self-closing": "off",
      "vue/html-end-tags": "off",
      "vue/html-indent": "off",
      "vue/multiline-html-element-content-newline": "off",
      "vue/singleline-html-element-content-newline": "off",
      // vue3-essential
      "vue-pug/no-parsing-error": "error",
      // vue3-strongly-recommended
      "vue-pug/no-pug-control-flow": "warn",
    },
  },
)

[IlyaSemenov]

I swear I searched for "flat" before submitting the issue...but somehow I overlooked #26.