chore: Migrate ESLint configuration file to new flat format. (#8675)

* chore: rename .eslintrc.js to eslint.config.js

* chore: Rename eslint.config.js to eslint.config.mjs.

* refactor: Migrate ESLint config to new flat format.

* chore: Remove old per-directory and global ignore ESLint config files.

* fix: Allowlist JSDoc tag aliases.

* fix: Don't require @license in tests/*.

* fix: Add NodeJS globals to several files that run under Node.

* chore: Remove now-unneeded ESLint directives in core.

* chore: Remove invalid/unneeded ESLint directives.

* fix: Fix invalid use of `await` outside of an `async` function.

* fix: Improve screenshot error message.

* fix: Update ESLint config file to not warn on existing violations.

* chore: Remove suppressions of rules that weren't triggering.

* chore: Fix package-lock.json.

tests/browser/.eslintrc.json

@@ -1,29 +0,0 @@
-{
-  "env": {
-    "browser": true,
-    "mocha": true,
-    "node": true
-  },
-  "globals": {
-    "chai": false,
-    "sinon": false
-  },
-  "rules": {
-    "no-unused-vars": ["off"],
-    // Allow uncommented helper functions in tests.
-    "require-jsdoc": ["off"],
-    "prefer-rest-params": ["off"],
-    "no-invalid-this": ["off"],
-    "valid-jsdoc": [
-      "error",
-      {
-        "requireReturnType": false,
-        "requireParamType": false
-      }
-    ]
-  },
-  "extends": "../../.eslintrc.js",
-  "parserOptions": {
-    "sourceType": "module"
-  }
-}