Migrate to ESLint 9 (#1747)

* Migrate to ESLint 9 with flat config and @typescript-eslint v8

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* Fix testbed eslint config to include all three tsconfig paths

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* Replace @typescript-eslint/eslint-plugin + parser with typescript-eslint package

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---------

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Co-authored-by: rzhao271 <7199958+rzhao271@users.noreply.github.com>

Author: Copilot

.eslintignore

@@ -0,0 +1,10 @@
+// @ts-check
+'use strict';
+
+const { createConfig } = require('../eslint.config.base.js');
+
+module.exports = createConfig(
+	['./src/tsconfig.json'],
+	undefined,
+	{}
+);

.eslintrc.base.json

@@ -20,7 +20,7 @@
 		"clean": "rimraf lib",
 		"compile": "tsc -b ./tsconfig.json",
 		"watch": "tsc -b ./tsconfig.watch.json -w",
-		"lint": "eslint --ext ts src",
+		"lint": "eslint src",
 		"test": "node ../build/bin/symlink-tests.js && node lib/runTests.js",
 		"all": "npm run clean && npm run compile && npm run lint && npm run test",
 		"symlink:publish": "node ../build/bin/symlink-client-tests-publish.js",

.eslintrc.json

@@ -0,0 +1,10 @@
+// @ts-check
+'use strict';
+
+const { createConfig } = require('../eslint.config.base.js');
+
+module.exports = createConfig(
+	['src/browser/tsconfig.json', 'src/common/tsconfig.json', 'src/node/tsconfig.json'],
+	undefined,
+	{}
+);