deps: Bump eslint to v9 + migrate to flat config

Migrated the eslint config using `npx @eslint/migrate-config .eslintrc.json`, and did not work flawlessly, obviously

I had to include config to examples into the root config file.
Instead, we are able to delete `.eslintrc` that existed for each example directory though

`require-atomic-updates` (has been set to `off`) does not exist in the default configs, so removed the line

`@typescript-eslint/no-unnecessary-type-assertion` requires a type assertion, removed it for now

eslint.config.mjs

@@ -0,0 +1,90 @@
+// @ts-check
+
+import eslint from '@eslint/js';
+import tseslint from 'typescript-eslint';
+import html from 'eslint-plugin-html';
+import { FlatCompat } from '@eslint/eslintrc';
+
+const compat = new FlatCompat({});
+
+export default tseslint.config(
+  {
+    files: ['**/*.{js,mjs,ts}'],
+    ignores: [
+      'packages/*/docs/',
+      'packages/*/lib/',
+      'packages/*/node_modules/',
+      'packages/*/types/',
+
+      'packages/*/examples/**/*.{js,html}',
+
+      '**/jest.config.js',
+    ],
+    extends: [
+      eslint.configs.recommended,
+      tseslint.configs.recommended,
+    ],
+    rules: {
+      curly: ['error', 'multi-line'],
+
+      'no-console': ['error', {
+        allow: ['info', 'warn', 'error'],
+      }],
+
+      '@typescript-eslint/naming-convention': ['error', {
+        selector: 'default',
+        format: ['camelCase'],
+      }, {
+        selector: 'variable',
+        format: ['camelCase', 'UPPER_CASE'],
+        leadingUnderscore: 'allow',
+      }, {
+        selector: 'typeLike',
+        format: ['PascalCase'],
+      }, {
+        selector: 'memberLike',
+        modifiers: ['public'],
+        format: ['camelCase', 'UPPER_CASE'],
+        leadingUnderscore: 'forbid',
+      }, {
+        selector: 'memberLike',
+        modifiers: ['protected'],
+        format: ['camelCase', 'UPPER_CASE'],
+        leadingUnderscore: 'require',
+      }, {
+        selector: 'memberLike',
+        modifiers: ['private'],
+        format: ['camelCase', 'UPPER_CASE'],
+        leadingUnderscore: 'require',
+      }, {
+        selector: 'enumMember',
+        format: ['PascalCase'],
+      }, {
+        selector: 'import',
+        format: ['camelCase', 'PascalCase', 'UPPER_CASE'],
+      }],
+
+      '@typescript-eslint/no-unused-vars': ['warn', {
+        args: 'none',
+      }],
+
+      '@typescript-eslint/no-explicit-any': 'off',
+      '@typescript-eslint/no-non-null-assertion': ['off'],
+    },
+  },
+
+  {
+    files: ['packages/*/examples/**/*.{js,html}'],
+    extends: [
+      eslint.configs.recommended,
+      ...compat.extends('mdcs'),
+    ],
+    plugins: {
+      html,
+    },
+    rules: {
+      'padded-blocks': 'off',
+      'no-unused-vars': 'off',
+    },
+  },
+);

package.json

@@ -19,17 +19,20 @@
     "prepare": "husky"
   },
   "devDependencies": {
+    "@eslint/eslintrc": "^3.2.0",
+    "@eslint/js": "^9.17.0",
     "@gltf-transform/core": "^4.0.8",
     "@types/jest": "^29.5.12",
     "@types/node": "^22.7.5",
     "@typescript-eslint/eslint-plugin": "^7.9.0",
     "@typescript-eslint/parser": "^7.9.0",
     "cross-env": "^7.0.2",
     "esbuild": "^0.24.0",
-    "eslint": "8",
+    "eslint": "^9.17.0",
     "eslint-config-mdcs": "^5.0.0",
     "eslint-config-prettier": "^9.1.0",
-    "eslint-plugin-html": "^8.1.1",
+    "eslint-plugin-html": "^8.1.2",
+    "globals": "^15.14.0",
     "husky": "^9.1.5",
     "jest": "^29.7.0",
     "lerna": "^8.1.8",
@@ -39,7 +42,8 @@
     "ts-jest": "^29.1.2",
     "ts-node": "^10.9.2",
     "typedoc": "^0.26.9",
-    "typescript": "^5.4.5"
+    "typescript": "^5.4.5",
+    "typescript-eslint": "^8.18.2"
   },
   "name": "three-vrm"
 }