chore: Migrate to ESLint flat config

Renamed `.eslintrc.js` to `eslint.config.js` and removed `.eslintignore`. Updated various dependencies including ESLint, eslint-config-airbnb-base, and eslint-plugin-import. Improved configuration structure to support jest and handle ignore patterns differently.

Author: kudoh

.eslintignore

@@ -0,0 +1,22 @@
+const airbnbBase = require('eslint-config-airbnb-base');
+const jest = require('eslint-plugin-jest');
+
+module.exports = [
+  {
+    ignores: ['!.eleventy.js', 'example']
+  },
+  {
+    rules: {
+      ...airbnbBase.rules,
+      'no-param-reassign': 'off',
+      'no-undef': 'off',
+    },
+  },
+  {
+    files: ['test/**'],
+    ...jest.configs['flat/recommended'],
+    rules: {
+      ...jest.configs['flat/recommended'].rules,
+    },
+  }
+]