ESLint Flat Config (#22221)

* Flat config file

* Plugin

* prettier

* Set eslint to latest version (non dev)

* yarn dedupe

* push changes from eslint type pr

* dedupe

eslint.config.mjs

@@ -0,0 +1,163 @@
+import unusedImports from "eslint-plugin-unused-imports";
+import globals from "globals";
+import tsParser from "@typescript-eslint/parser";
+import path from "node:path";
+import { fileURLToPath } from "node:url";
+import js from "@eslint/js";
+import { FlatCompat } from "@eslint/eslintrc";
+
+const __filename = fileURLToPath(import.meta.url);
+const __dirname = path.dirname(__filename);
+const compat = new FlatCompat({
+  baseDirectory: __dirname,
+  recommendedConfig: js.configs.recommended,
+  allConfig: js.configs.all,
+});
+
+export default [
+  ...compat.extends(
+    "airbnb-base",
+    "airbnb-typescript/base",
+    "plugin:@typescript-eslint/recommended",
+    "plugin:wc/recommended",
+    "plugin:lit/all",
+    "plugin:lit-a11y/recommended",
+    "prettier"
+  ),
+  {
+    plugins: {
+      "unused-imports": unusedImports,
+    },
+
+    languageOptions: {
+      globals: {
+        ...globals.browser,
+        __DEV__: false,
+        __DEMO__: false,
+        __BUILD__: false,
+        __VERSION__: false,
+        __STATIC_PATH__: false,
+        __SUPERVISOR__: false,
+        Polymer: true,
+      },
+
+      parser: tsParser,
+      ecmaVersion: 2020,
+      sourceType: "module",
+
+      parserOptions: {
+        ecmaFeatures: {
+          modules: true,
+        },
+
+        project: "./tsconfig.json",
+      },
+    },
+
+    settings: {
+      "import/resolver": {
+        webpack: {
+          config: "./webpack.config.cjs",
+        },
+      },
+    },
+
+    rules: {
+      "class-methods-use-this": "off",
+      "new-cap": "off",
+      "prefer-template": "off",
+      "object-shorthand": "off",
+      "func-names": "off",
+      "no-underscore-dangle": "off",
+      strict: "off",
+      "no-plusplus": "off",
+      "no-bitwise": "error",
+      "comma-dangle": "off",
+      "vars-on-top": "off",
+      "no-continue": "off",
+      "no-param-reassign": "off",
+      "no-multi-assign": "off",
+      "no-console": "error",
+      radix: "off",
+      "no-alert": "off",
+      "no-nested-ternary": "off",
+      "prefer-destructuring": "off",
+      "no-restricted-globals": [2, "event"],
+      "prefer-promise-reject-errors": "off",
+      "import/prefer-default-export": "off",
+      "import/no-default-export": "off",
+      "import/no-unresolved": "off",
+      "import/no-cycle": "off",
+
+      "import/extensions": [
+        "error",
+        "ignorePackages",
+        {
+          ts: "never",
+          js: "never",
+        },
+      ],
+
+      "no-restricted-syntax": ["error", "LabeledStatement", "WithStatement"],
+      "object-curly-newline": "off",
+      "default-case": "off",
+      "wc/no-self-class": "off",
+      "no-shadow": "off",
+      "@typescript-eslint/camelcase": "off",
+      "@typescript-eslint/ban-ts-comment": "off",
+      "@typescript-eslint/no-use-before-define": "off",
+      "@typescript-eslint/no-non-null-assertion": "off",
+      "@typescript-eslint/no-explicit-any": "off",
+      "@typescript-eslint/explicit-function-return-type": "off",
+      "@typescript-eslint/explicit-module-boundary-types": "off",
+      "@typescript-eslint/no-shadow": ["error"],
+
+      "@typescript-eslint/naming-convention": [
+        "off",
+        {
+          selector: "default",
+          format: ["camelCase", "snake_case"],
+          leadingUnderscore: "allow",
+          trailingUnderscore: "allow",
+        },
+        {
+          selector: ["variable"],
+          format: ["camelCase", "snake_case", "UPPER_CASE"],
+          leadingUnderscore: "allow",
+          trailingUnderscore: "allow",
+        },
+        {
+          selector: "typeLike",
+          format: ["PascalCase"],
+        },
+      ],
+
+      "@typescript-eslint/no-unused-vars": "off",
+
+      "unused-imports/no-unused-vars": [
+        "error",
+        {
+          vars: "all",
+          varsIgnorePattern: "^_",
+          args: "after-used",
+          argsIgnorePattern: "^_",
+          ignoreRestSiblings: true,
+        },
+      ],
+
+      "unused-imports/no-unused-imports": "error",
+      "lit/attribute-names": "warn",
+      "lit/attribute-value-entities": "off",
+      "lit/no-template-map": "off",
+      "lit/no-native-attributes": "warn",
+      "lit/no-this-assign-in-render": "warn",
+      "lit-a11y/click-events-have-key-events": ["off"],
+      "lit-a11y/no-autofocus": "off",
+      "lit-a11y/alt-text": "warn",
+      "lit-a11y/anchor-is-valid": "warn",
+      "lit-a11y/role-has-required-aria-attrs": "warn",
+      "@typescript-eslint/consistent-type-imports": "error",
+      "@typescript-eslint/no-import-type-side-effects": "error",
+    },
+  },
+];