build: add eslint.config.cjs for JavaScript targets on ESLint v8

Add flat config alongside the legacy `.eslintrc.*` configuration:

- Create `eslint.config.cjs` with base JS config, override blocks
  for benchmarks, examples, tests, and REPL namespace files
- Inline stdlib rules as a runtime plugin object via
  `lib/node_modules/@stdlib/_tools/eslint/rules/scripts/plugin.js`
- Reuse existing rule definitions from `etc/eslint/rules/`
- Separate non-clonable rule options (remark plugin instances) from
  the main rules object to work around `structuredClone` limitations
  in ESLint's flat config internals
- Add `globals` package for flat config environment definitions
- Legacy `.eslintrc.*` files remain in place as the default workflow

Usage (requires ESLint v8 with flat config enabled):

    ESLINT_USE_FLAT_CONFIG=true npx eslint -c eslint.config.cjs <file>

Note: files under `lib/node_modules/` are hard-ignored by ESLint v8
flat config. Full `lib/` linting via flat config requires ESLint v9,
which allows `!**/node_modules/` ignore overrides.

Ref: stdlib-js/metr-issue-tracker#54

Author: Planeshifter

eslint.config.cjs

@@ -0,0 +1,162 @@
+/**
+* @license Apache-2.0
+*
+* Copyright (c) 2026 The Stdlib Authors.
+*
+* Licensed under the Apache License, Version 2.0 (the "License");
+* you may not use this file except in compliance with the License.
+* You may obtain a copy of the License at
+*
+*    http://www.apache.org/licenses/LICENSE-2.0
+*
+* Unless required by applicable law or agreed to in writing, software
+* distributed under the License is distributed on an "AS IS" BASIS,
+* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+* See the License for the specific language governing permissions and
+* limitations under the License.
+*/
+
+'use strict';
+
+// MODULES //
+
+var globals = require( 'globals' );
+var pluginN = require( 'eslint-plugin-n' );
+var pluginCspell = require( '@cspell/eslint-plugin' );
+var pluginJsdoc = require( 'eslint-plugin-jsdoc' );
+var stdlibPlugin = require( './lib/node_modules/@stdlib/_tools/eslint/rules/scripts/plugin.js' );
+var allRules = require( './etc/eslint/rules' );
+var overrides = require( './etc/eslint/overrides' );
+
+
+// VARIABLES //
+
+var restrictedSyntaxConfig = overrides[ 1 ].rules[ 'no-restricted-syntax' ];
+var nonClonableRules = {};
+var rules = {};
+var val;
+var key;
+var i;
+
+
+// FUNCTIONS //
+
+/**
+* Tests whether a value can be structured-cloned.
+*
+* @private
+* @param {*} value - value to test
+* @returns {boolean} boolean indicating whether the value is clonable
+*/
+function isClonable( value ) {
+	try {
+		// eslint-disable-next-line n/no-unsupported-features/es-builtins
+		if ( typeof structuredClone === 'function' ) {
+			structuredClone( value );
+		}
+		return true;
+	} catch ( e ) {
+		return false;
+	}
+}
+
+
+// MAIN //
+
+// Separate rules that contain non-clonable values (e.g., remark plugin
+// instances) because ESLint flat config internally clones config objects:
+for ( key in allRules ) {
+	val = allRules[ key ];
+	if ( isClonable( val ) ) {
+		rules[ key ] = val;
+	} else {
+		nonClonableRules[ key ] = val;
+	}
+}
+
+module.exports = [
+	// Global ignores:
+	{
+		'ignores': [
+			'**/build/',
+			'**/reports/',
+			'dist/',
+			'.git*'
+		]
+	},
+
+	// Base JavaScript config:
+	{
+		'files': [ '**/*.js' ],
+		'languageOptions': {
+			'ecmaVersion': 6,
+			'sourceType': 'script',
+			'globals': {
+				...globals.browser,
+				...globals.node,
+				...globals.commonjs,
+				...globals.worker
+			}
+		},
+		'plugins': {
+			'n': pluginN,
+			'jsdoc': pluginJsdoc,
+			'@cspell': pluginCspell,
+			'stdlib': stdlibPlugin
+		},
+		'rules': rules
+	},
+
+	// REPL namespace files:
+	{
+		'files': [ '**/lib/node_modules/@stdlib/**/lib/[a-z].js' ],
+		'rules': {
+			'stdlib/repl-namespace-order': 'error'
+		}
+	},
+
+	// Benchmarks:
+	{
+		'files': [ '**/benchmark/**/*.js' ],
+		'rules': {
+			'no-new-wrappers': 'warn',
+			'max-lines': [ 'warn', {
+				'max': 1000,
+				'skipBlankLines': true,
+				'skipComments': true
+			}],
+			'jsdoc/require-jsdoc': 'off',
+			'no-restricted-syntax': restrictedSyntaxConfig
+		}
+	},
+
+	// Examples:
+	{
+		'files': [ '**/examples/**/*.js' ],
+		'rules': {
+			'no-console': 'off',
+			'vars-on-top': 'off',
+			'jsdoc/require-jsdoc': 'off',
+			'stdlib/jsdoc-private-annotation': 'off',
+			'stdlib/require-order': 'off',
+			'stdlib/require-file-extensions': 'off',
+			'no-restricted-syntax': restrictedSyntaxConfig
+		}
+	},
+
+	// Tests:
+	{
+		'files': [ '**/test/**/*.js' ],
+		'rules': {
+			'no-empty-function': 'off',
+			'jsdoc/require-jsdoc': 'off',
+			'no-undefined': 'off',
+			'max-lines': [ 'warn', {
+				'max': 1000,
+				'skipBlankLines': true,
+				'skipComments': true
+			}],
+			'no-restricted-syntax': restrictedSyntaxConfig
+		}
+	}
+];

package.json

@@ -139,12 +139,13 @@
     "doctrine": "^3.0.0",
     "editorconfig-checker": "^6.0.0",
     "envify": "^4.0.0",
-    "@stylistic/eslint-plugin-ts": "^4.2.0",
+    "@stylistic/eslint-plugin-ts": "^2.13.0",
     "eslint": "^8.57.0",
     "eslint-plugin-n": "^17.17.0",
     "eslint-plugin-expect-type": "^0.2.3",
     "eslint-plugin-import": "^2.29.0",
     "eslint-plugin-jsdoc": "^46.8.2",
+    "globals": "^16.1.0",
     "exorcist": "^2.0.0",
     "factor-bundle": "^2.5.0",
     "gh-pages": "git+https://github.com/Planeshifter/gh-pages.git#main",