refactor: Update ESLint configuration and remove unused files

- Deleted .env.example .eslintignore and .eslintrc.cjs files as they are no longer needed.
- Introduced a new eslint.config.js file

Author: ryota-murakami

.env.example

@@ -0,0 +1,4 @@
+import { defineConfig } from 'eslint/config'
+import tsPrefixer from 'eslint-config-ts-prefixer'
+
+export default defineConfig([...tsPrefixer])

.eslintignore

@@ -8,8 +8,8 @@
     "prepare": "husky",
     "prettier": "prettier --ignore-unknown --write .",
     "test": "vitest run",
-    "lint": "eslint . --ext .ts,.tsx,.js,jsx,cjs,mjs",
-    "lint:fix": "eslint . --ext .ts,.tsx,.js,jsx,cjs,mjs --fix",
+    "lint": "eslint . -c eslint.config.js",
+    "lint:fix": "eslint . -c eslint.config.js --fix",
     "push-release-commit": "push-release-commit"
   },
   "repository": {
@@ -45,11 +45,8 @@
     "@typescript-eslint/parser": "^8.32.0",
     "@vitest/coverage-v8": "3.1.4",
     "@vitest/ui": "^3.1.2",
-    "eslint": "8.57.0",
-    "eslint-config-ts-prefixer": "2.0.0-beta.1",
-    "eslint-import-resolver-typescript": "^4.3.4",
-    "eslint-plugin-import": "^2.31.0",
-    "eslint-plugin-prettier": "^5.4.0",
+    "eslint": "^9.28.0",
+    "eslint-config-ts-prefixer": "^3.0.2",
     "husky": "^9.1.7",
     "lint-staged": "^16.0.0",
     "prettier": "^3.5.3",