Update: use eslint flat config

.eslintrc.js and .eslintignore are deprecated and the new flat config system is preferred. The CLI also doesn't
support --ext flag with the flat config system.

How the new config is generated:
https://eslint.org/docs/latest/use/configure/migration-guide#migrate-your-config-file
Reference for cli flag changes:
https://eslint.org/docs/latest/use/configure/migration-guide#cli-flag-changes

eslint.config.mjs

@@ -0,0 +1,75 @@
+import header from "eslint-plugin-header";
+import globals from "globals";
+import path from "node:path";
+import { fileURLToPath } from "node:url";
+import js from "@eslint/js";
+import { FlatCompat } from "@eslint/eslintrc";
+
+const __filename = fileURLToPath(import.meta.url);
+const __dirname = path.dirname(__filename);
+const compat = new FlatCompat({
+  baseDirectory: __dirname,
+  recommendedConfig: js.configs.recommended,
+  allConfig: js.configs.all,
+});
+
+export default [
+  {
+    ignores: [
+      "**/out",
+      "**/poky",
+      "**/.vscode-test",
+      "__mocks__/vscode.ts",
+      "**/jest.config.js",
+      "**/eslint.config.mjs",
+    ],
+  },
+  ...compat.extends(
+    "standard-with-typescript",
+    "plugin:deprecation/recommended"
+  ),
+  {
+    plugins: {
+      header,
+    },
+
+    languageOptions: {
+      globals: {
+        ...globals.browser,
+      },
+
+      ecmaVersion: "latest",
+      sourceType: "module",
+    },
+
+    rules: {
+      "header/header": [
+        2,
+        "block",
+        [
+          " --------------------------------------------------------------------------------------------",
+          {
+            pattern: " \\* Copyright \\(c\\) .*\\. All rights reserved\\.",
+            template:
+              " * Copyright (c) 2023 Savoir-faire Linux. All rights reserved.",
+          },
+          " * Licensed under the MIT License. See License.txt in the project root for license information.",
+          " * ------------------------------------------------------------------------------------------ ",
+        ],
+        2,
+      ],
+    },
+  },
+  {
+    files: ["**/*.ts"],
+
+    languageOptions: {
+      globals: {
+        ...globals.node,
+      },
+
+      ecmaVersion: 5,
+      sourceType: "commonjs",
+    },
+  },
+];

package.json

@@ -894,7 +894,7 @@
 		"clean:client": "rm -fr ./client/node_modules ./client/out client/tsconfig.tsbuildinfo client/.vscode-test",
 		"clean:lib": "rm -fr ./lib/node_modules",
 		"clean": "npm run clean:lib && npm run clean:server && npm run clean:client && rm -fr node_modules integration-tests/out integration-tests/project-folder/build* .vscode-test .eslintcache resources coverage ./out *.vsix",
-		"lint": "eslint . --ext js,ts --cache",
+		"lint": "eslint . --cache",
 		"jest": "jest",
 		"test": "npm run jest && npm run test:integration && npm run test:grammar",
 		"test:integration": "xvfb-run node ./integration-tests/out/runTest.js",