Upgrade eslint to v9 (#517)

.prettierrc.d.ts

@@ -0,0 +1,4 @@
+import type { Options } from 'prettier';
+
+declare const options: Options;
+export = options;

.prettierrc.js

@@ -0,0 +1,34 @@
+// @ts-check
+
+/**
+ * @type {import('prettier').Config}
+ */
+export default {
+  plugins: ['prettier-plugin-organize-imports', 'prettier-plugin-packagejson'],
+  singleQuote: true,
+  trailingComma: 'all',
+  overrides: [
+    {
+      files: '*.json5',
+      options: {
+        parser: 'json5',
+        quoteProps: 'preserve',
+        singleQuote: false,
+        trailingComma: 'none',
+      },
+    },
+    {
+      files: '*.md',
+      options: {
+        // @ts-expect-error: known property
+        organizeImportsSkipDestructiveCodeActions: true,
+      },
+    },
+    {
+      files: 'package.json',
+      options: {
+        packageSortOrder: ['name', 'version', 'description', 'scripts'],
+      },
+    },
+  ],
+};

.vscode/settings.json

@@ -31,6 +31,9 @@
     "vuepress",
     "yaml"
   ],
+  "eslint.options": {
+    "flags": ["unstable_ts_config"]
+  },
   "eslint.validate": ["javascript", "typescript"],
   "files.associations": {
     "*.json5": "jsonc"