Fix CI: upgrade eslint-plugin-github to 6.1.1, migrate to flat config

Author: Copilot

.eslintignore

@@ -0,0 +1,48 @@
+import githubPlugin from 'eslint-plugin-github'
+import {fixupPluginRules} from '@eslint/compat'
+import customElementsPlugin from 'eslint-plugin-custom-elements'
+import globals from 'globals'
+
+const githubFlatConfigs = githubPlugin.getFlatConfigs()
+
+const customElementsRules = Object.fromEntries(
+  Object.keys(customElementsPlugin.rules).map(r => [`custom-elements/${r}`, 'error']),
+)
+
+export default [
+  {ignores: ['dist']},
+  githubFlatConfigs.browser,
+  githubFlatConfigs.recommended,
+  ...githubFlatConfigs.typescript,
+  {
+    plugins: {
+      'custom-elements': fixupPluginRules(customElementsPlugin),
+    },
+    languageOptions: {
+      globals: {
+        FileAttachmentElement: 'readonly',
+      },
+    },
+    rules: {
+      ...customElementsRules,
+      '@typescript-eslint/no-explicit-any': 'off',
+      'custom-elements/tag-name-matches-class': ['error', {suffix: 'Element'}],
+    },
+  },
+  {
+    files: ['test/**/*.js'],
+    languageOptions: {
+      globals: {
+        ...globals.mocha,
+        ...globals.chai,
+      },
+    },
+    rules: {
+      'github/unescaped-html-literal': 'off',
+      'github/no-inner-html': 'off',
+      'import/extensions': 'off',
+      'import/no-unresolved': 'off',
+      'one-var': 'off',
+    },
+  },
+]